Variations in serine transhydroxymethylase (STHM) activity as a function of the addition of certain nutrilites to the growth medium of cells ofEscherichia coliK12 have been studied. STHM activity was decreased by growth in (a) glycine at high concentrations, and (b) a mixture of C1-derived metabolites (histidine, methionine, adenine, guanine, thymidine) at moderate concentrations. Two hypotheses are explored. The regulation of STHM synthesis can be explained in terms of variations in the intracellular concentration of glycine alone, or together with variations in the concentration of a C1intermediate.