In order to get information about the mechanism of action of alkylating agents, survival was studied after treatment with methyl methanesulfonate, in an arginine-requiring strain of the unicellular green algaChlamydomonas reinhardi. Methylated cells are susceptible to further degradation if they are allowed to stand at ordinary temperature and are differentially sensitive to heating in buffer at 40° C. The heat sensitivity of the cells is greatly reduced if they are incubated overnight in the dark, irrespective of the medium (buffer, minimal medium-supplemented or not-or complete medium). The results are interpreted by assuming that some primary damage remains in the cell after the treatment and that this may be converted into actual damage. This conversion is favoured by drastic conditions of post-treatment, immediately and before the occurrence of recovery from primary damage.