Recently, several excellent reviews appeared /9, 12, 15, 29/ summarizing new findings of the last years and describing the topic in the new interconnections. The signalling systems through which changes in environmental conditions affecting the growth of microorganism are sensed, transmitted and converted into mechanisms controlling the production of antibiotic can now be investigated with the techniques of molecular genetics. Evidence has been accumulated that demonstrated the key roles played by diffusible molecules in regulating cellular differentiation even among prokaryotic microorganisms. This is exemplified by A-factor and its analogues, which act as autoregulators for morphological differentiation and secondary metabolism in Streptomyces. In our article we have concentrated on the physiological changes, which can occur during the growth in natural environment or during cultivation under laboratory conditions. Recent evidence for the presence of molecular signalling systems in Streptomyces is reviewed, along with the inherent implications. The constitution of the metabolic type during the first hours of cultivation has been previously reviewed /36/.