Homocysteine has been shown to increase glutathione levels in C3H/10T1/2 Cl 8 cells. The present paper confirms that this increase was specific for non-dividing cells. Several other thiols and disulfides, including cysteamine, mercaptoethanol and dithioerythritol, also increased glutathione, but the specificity for quiescent non-dividing cells was confined to homocysteine only. Cysteamine was most efficient, increasing glutathione 5-fold in confluent, non-dividing cells, and 3.2-fold in exponentially growing Cl 8 cells. The results indicate that the increase in glutathione was not specific for homocysteine or other cysteine generating agents, but rather related to the presence of potential thiol, either in free form, as thiolactone or in its oxidized, disulfide form. The effect of the glutathione synthesis inhibitor BSO was investigated in detail in both C3H/10T1/2 Cl 8 cells and in R1.1. mouse lymphoma cells. Twenty-four hours after addition of 20 microM BSO to exponential growing Cl 8 cells the glutathione content was reduced to 5.5%, with minimal toxic effect. To achieve the same GSH depleting effect on exponential growing R1.1. cells, the BSO concentration had to be increased to 50 microM, which had a slight, but distinct growth inhibitory effect on the lymphoma cells. Based on these data, the possibility that glutathione mediated homocysteine production was investigated in part by depleting the cells of glutathione and determining the homocysteine export rate as a measure of the intracellular production of the metabolite. The results showed that glutathione depletion by BSO had no effect on the homocysteine export rate in Cl 8 cells, while in R1.1. cells a moderate decrease in homocysteine export rate accompanied by a slight, but distinct decrease in growth rate, was observed when the cells were depleted of glutathione. In addition, these data indicate that BSO did not interfere with the overall transmethylation rate, and this observation supports the view of BSO as a specific inhibitor of GSH synthesis. A general difference between the homocysteine export rate in Cl 8 and R1.1. cells was observed. The former demonstrated a decreasing export rate during exponential growth, while the latter showed an initial decrease and then a slight increase in homocysteine export rate.