The coastal environment pollution is becoming more serious with the development of industry. The extensive use of antibiotics in aquaculture and increasing land-based input result in pollution of coastal waters, which may affect the growth and physiology of marine macroalgae, the important primary producers in this area. They are distributed in the intertidal and subtidal zones of the coastal area that are most populated. The red macroalga Gracilaria lemaneiformis (Bory) Weber-van Bosse, which has higher content and quality for agar production compared to other species in the Gracilaria genera, is being commercially cultivated in China and used for agar production and foodstuff. As a result of the fast growth rate, the plantings of G. lemaneiformis are very abroad in the alongshore of sea in South China. This paper investigates the responses of growth, photosynthesis, respiration and biochemical components with exposure to different concentrations of two common antibiotics (chloramphenicol and benzylpenicillin sodium salt) in G. lemaneiformis, in order to evaluate the effects of antibiotics on the physiology of cultivated seaweed species. Five different concentrations of antibiotics are adopted (the concentrations of chloramphenicol are as follow: 0, 10, 50, 100, 250mg/L, and those of benzylpenicillin sodium salt are 0, 10, 50, 250, 500, 1000mg/L). G. lemaneiformis was cultured under these different concentrations of the two antibiotics for seven days. Then a variety of physiological and biochemical indicators were measured. The results showed that the growth of G. lemaneiformis was negatively affected by both antibiotics. The relative growth rate of G. lemaneiformis decreased with the increasing of antibiotics, with the sensitivity being more in chloramphenicol than benzylpenicillin sodium salt. In the treatments of chloramphenicol, photosynthesis, effective quantum efficiencies (Yield), phycobiliproteins (including phycoerythrin and phycocyanin), and the contents of soluble proteins were sharply reduced with the increasing concentrations of chloramphenicol. However, respiration rates of G. lemaneiformis were increased with the chloramphenicol treatment. The contents of pigments in G. lemaneiformis were not significantly affected by chloramphenicol. In the treatments of benzylpenicillin sodium salt, both of photosynthesis and Yield were progressively decreased with the increasing concentrations of benzylpenicillin sodium salt, whereas the contents of pigments exhibited the opposite trend. The contents of phycobiliproteins and soluble proteins were not singificantly affected by the treatment of benzylpenicillin sodium salt. The results suggested that reduced growth was mainly due to the reduced photosynthesis rates and reduced synthesis of proteins in case of chloramphenicol, or could be ascribed to the increased respiration rates in case of benzylpenicillin sodium salt. The different physiological and biochemical responses of G. lemaneiformis to chloramphenicol and benzylpenicillin sodium salt resulted from different action mechanism of these two antibiotics. Chloramphenicol would mainly interfere the binding of t-RNA and the 50S subunit, then block the formation of the new polypeptide chain, and finally inhibit protein synthesis. However, benzylpenicillin sodium salt would play a very important role in interfering the synthesis of cell wall. Our results suggested that chloramphenicol might be more suitable as a selection pressure in genetic engineering breeding, since G. lemaneiformis was more sensitivity to chloramphenicol than benzylpenicillin sodium salt.