Summary Sodium selenite (Na2SeO3) and some sulphydryl (-SH) compounds, including the amino acid cysteine protect Chlorella against the sub-lethal effects of mercuric chloride (HgCl2) and phenylmercuric acetate (PMA). However just the addition of selenium and the sulphur compounds to the culture medium containing zinc chloride (ZnCl2), HgCl2 or PMA provided little or no protection. But when Chlorella was incubated for six days in selenium and the sulphydryl compounds prior to metal treatments, then cells were markedly protected against mercury toxicity (but not zinc toxicity) as measured by increased cell division rates. Cells incubated in methionine (-SCH3) were only slightly protected, and cells incubated in the disulphide compound, cystine (-SS) were not protected at all. The increase in rates of cell division (protection) was strongly correlated to the amount of sulphydryl groups in the cells. Physiological implications of the work are discussed.