Methionine adenosyltransferase activity in normal diploid and simian virus 40 (SV40)-transformed human fibroblasts increased severalfold when cell monolayers were cultured in medium deficient in L-methionine. This increase in methionine adenosyltransferase activity required RNA and protein syntheses and probably represented a derepression of the enzyme's biosynthesis. Furthermore, studies with RNA synthesis inhibitors suggested that the regulation of this enzyme activity in human fibroblasts involved posttranscriptional mechanisms. The inclusion of homocysteine thiolactone, a metabolic precursor of methionine, in the methionine-deficient medium inhibited the derepression in normal human fibroblasts but augmented the derepression in fully transformed fibroblasts. These differences in derepression patterns thus appeared related to altered metabolism of homocysteine and/or methionine in SV40-transformed human fibroblasts and as such may serve as a transformation marker in SV40-transformed cells.