The effect of methyl mercuric chloride on the activity of phospholipid synthesis of rat lymph node lymphocytes was compared with that of sulfhydryl inhibitors. Measurement of the radioactivity of [14C]oleic acid and [14C]acetate incorporated into lecithin of cells during short-term incubation showed that all the inhibitors tested similarly reduced the incorporation. However, methyl mercuric chloride (MMC) was the strongest inhibitor, being effective at 4 microM and causing more than 80% decrease at 20 microM. Inhibition by the sulfhydryl inhibitors, at less than 40 microM, ranked as follows: N-ethylmaleimide greater than alpha-chloroacetophenone greater than hydroquinone greater than iodoacetamide. MMC also obstructed the enhancement by phytohemagglutinin of [14C]oleic acid incorporation into lecithin. MMC was effective at 2 microM, while the other agents had little or no effect at this concentration. Further investigation suggested that inhibition of phospholipid synthesis did not depend on reduced incorporation of oleic acid into the cellular membrane but on decreased turnover of the fatty acid into phospholipids after the incorporation. The viability of lymphocytes incubated with the agents was measured by trypan blue dye-exclusion test. More than 90% of the cells treated with MMC at a concentration as low as 20-40 microM died, but the SH inhibitors, including NEM which greatly inhibited the phospholipid synthesis, produced few cell deaths at these concentrations. These observations show that the SH inhibitors affect enzymes in phospholipid synthesis, whereas MMC not only inhibits the enzymes but kills cells.