The action of the nitrogen mustard di-(2-chloroethyl) methylamine hydrochloride (HN2) on the in vitro incorporation of labelled amino acids into the proteins of rat liver slices has been compared with its action on other biosynthetic processes in the liver. Uptake of 14C-glycine was found to be less sensitive to HN2 than was incorporation of 32P into ribonucleic acid. Liver slices were treated with different concentrations of HN2 and nuclear and cytoplasmic fractions isolated separately after incubation. Uptake of 14C-glycine by the proteins of these two fractions was similarly affected by HN2. The action of HN2 on 14C-phenylalanine incorporation and on p-aminohippuric acid synthesis by liver slices were then compared. At all concentrations of the inhibitor, essentially the same response was obtained with both systems. It has been concluded that the inhibition of amino acid incorporation produced by HN2 acting on liver slices is due to general inhibition of synthetic reactions rather than to a specific action on the mechanism of protein synthesis.