Several structural analogs of 4-methyl-3-hydroxyanthranilic acid (4-MHA) that had been established as substrates of the 4-MHA-activating enzyme from Streptomyces chrysomallus (Keller, U., Kleinkauf, H., and Zocher, R. (1984) Biochemistry 23, 1479-1484) were fed in short term labeling experiments to cultures of two actinomycin-producing streptomycetes. Besides inhibition of actinomycin synthesis, the addition of 4-methyl-3-hydroxybenzoic acid, 3-hydroxybenzoic acid, 4-aminobenzoic acid, or 4-methyl-3-methoxy-benzoic acid induced the formation of novel compounds. The data indicate that these compounds are structural analogs of 4-MHA pentapeptide lactones, which are the most probable precursors of actinomycins (Katz, E. (1967) in Antibiotics II (Gottlieb, D., and Shaw, P. D., eds) pp. 276-341, Springer-Verlag, New York). Since the structural analogs of 4-MHA are missing the o-aminophenol configuration, the corresponding acyl pentapeptide lactones cannot react with each other to give phenoxazines. Therefore, they accumulate and thus become detectable. Feeding cultures with 4-MHA resulted in an inhibition of actinomycin synthesis apparently at the level of acyl pentapeptide lactone synthesis. In such experiments, the formation of pentapeptide lactones, which are most likely derived from 4-MHA pentapeptide lactones, could be detected. The results provide experimental evidence that the biosynthesis of actinomycins proceeds via the 4-MHA-pentapeptide lactones.