Enterotoxigenic Escherichia coli or Vibrio cholerae 569B (Inaba) grown in the presence of the antibiotic lincomycin, an inhibitor of protein synthesis, produced elevated levels of heat-labile enterotoxin or choleragen, respectively, as assayed by both vascular permeability factor and capacity to elicit fluid accumulation in rabbit ileal loops. This induction of enterotoxin did not reflect either a coupling of lincomycin resistance with increased enterotoxigenicity or an effect of lincomycin on cellular release of enterotoxin, since spontaneously isolated lincomycin-resistant mutants of both E. coli and V. cholerae still required lincomycin for induction, and large increases in E. coli permeability factor activity were found intracellularly as well as extracellularly. After the period of exponential growth, E. coli became refractory to induction by lincomycin, although most of the induced enterotoxin activity appeared only after this period. No increase in copy number of the enterotoxin plasmid in E. coli 711 (P307) was found in induced cells by analysis of deoxyribonucleic acid reassociation kinetics. These and other data suggest that synthesis of enterotoxin, or at least its accumulation, is normally limited by cellular factors whose synthesis is preferentially inhibited by lincomycin. A possible connection between this phenomenon and lincomycin-associated diarrhea is considered.