Ehrlich ascites tumor cells (EAT) could not proliferate in a basal medium (BM). L-Cysteine (L-CYS) added to BM in a concentration of 0.5 to 2 mM could promote the growth of EAT. At an unusual concentration of L-CYS (1 mM), the growth-rate was 233% (n = 50) at 72 hr after EAT transplantation. A similar growth-rate was obtained when EAT was transplanted 24 hr after addition of L-CYS to BM, although the free SH content in L-CYS-added BM at this time decreased to the level of that in BM. S-Methyl- and S-benzyl-L-cysteine showed no growth-promoting effect. The metabolic products of L-CYS (L-cysteine sulfinic acid, hypotaurine and taurine) showed a lower effect or none, but L-cystine showed the same activity as L-CYS. The EAT cultured in L-CYS-added BM was found to have an increased intracellular free SH and unchanged protein SH. These findings suggest that L-CYS added to BM is changed in a non-SH compound (possibly L-cystine) that is taken up into the cells and increases the intracellular free SH, resulting in the cell growth promotion.