m-Aminophenylboronic acid (APBA) inhibited the germination, growth and sporulation of Streptomyces griseus NRRL B-2682 in an age- and concentration-dependent manner in submerged and solid cultures. When added to cells or cell extracts it irreversibly inhibited NAD+-glycohydrolase and ADP-ribosyltransferase activity. ADP-ribosyltransferase was more sensitive, but inhibition was not complete, even in the presence of 10 mM APBA. The in vivo effects of the inhibitor correlated with its in vitro effect on ADP-ribosylation and on the profile of ADP-ribosylated endogenous proteins. The physiological importance of ADP-ribosyltransferase was supported by the observation that APBA strongly inhibited the growth of a non-sporulating and NAD+- glycohydrolase-negative mutant of the parental strain. The resistance of S. griseus NRRL B-2682 strains able to grow in the presence of APBA was due to permeability factors. A comparison of the ADP-ribosylated protein profiles of S. griseus NRRL B-2682 grown under various conditions showed similarities, but also specific differences. The results suggest that the ADP-ribosyltransferase of S. griseus NRRL B-2682 is an indispensable enzyme for growth and differentiation of the strain. It may regulate the activity of key enzymes or developmental proteins by responding to intra- and extracellular conditions.