Adenosine 3’:5’ cyclic monophosphate seems to regulate antibiotic biosynthesis and secondary metabolism in tylosin-producing cultures of Streptomyces fradiae C373.1. A dose-dependent response is observed by exogenous additions of dibutyryl cyclic AMP (cAMP), and is related to the nutritional status of the culture. Addition of cAMP to cultures growing in nutritionally lean media caused higher cumulative antibiotic titers and some cellular differentiation compared with the control. In nutritionally rich media, a qualitatively different behavior resulted : an almost instantaneous shift toward secondary metabolism occurred. The response is characterized by extensive cellular differentiation with little growth and only a trace of antibiotic production. The possible role of cyclic AMP in the regulation of tylosin biosynthesis and secondary metabolism and its relation to specific nutrient limitations in synthetic, defined media in Streptomyces fradiae is discussed.