The two species of phytoplankton used in this experiment were a diatom Phaeodactylum tricornutum Bobl and a green alga Chlordla vulgaris Beij.Both species took up 60Co. The concentration factor was constant with several activating concentrations in our experiments. However, there was a seasonal variation, probably due to difference in physiological states.There was a clear correlation between the accumulation of 60Co in the cell and the decrease of activity in the medium, which was in keeping with the incre...