The growth of 4 tumour-cell lines (Walker rat mammary carcinoma (W-256), a mouse lymphoma (TLX5), a mouse bladder carcinoma (MB) and a human bladder carcinoma (EJ) was much reduced when methionine in the culture medium was substituted by homocysteine. In contrast, a human embryonic fibroblast line grew equally well under such conditions. Although homocysteine alone was unable to support growth of W-256 it stimulated growth at low methionine concentrations. When W-256 was cultured for 24 h in medium containing homocysteine only, the extent of methylation of nucleic acids and the acid-soluble pool of methionine were decreased. However, under such conditions there was an increased methylase activity towards both endogenous substrate and E. coli tRNA. The effect of methionine removal was to cause a large increase in the Vmax value for methylation of tRNA, without any change in the Km value towards S-adenosyl-L-methionine (SAM). For both W-256 and TLX5, methionine deprivation caused a rapid inhibition of RNA biosynthesis, followed by inhibition of DNA synthesis, while protein synthesis tended to increase. This suggests that the inability of W-256 and TLX5 to survive and grow in methionine-deficient, homocysteine-supplemented medium is not due to insufficient methionine for protein biosynthesis, but may be related to an enhanced methylating activity of some tumour-cell lines.