L-p-Aminophenylalanine-α-14C:α-15N was shown to be a specific precursor of the p-nitrophenylserinol moiety of chloramphenicol produced by cultures of Streptomyces species. It was incorporated with only a small change in the 14C:15N ratio. Evidence was obtained for the presence of a small intracellular pool of p-aminophenylalanine. DL-threo-p-Aminophenylserine-carboxyl-14C and α-N-dichloroacetyl-Ls-threo-p-aminophenylserinol-hydroxymethyl-14C were also incorporated into the antibiotic without randomization of the label.