Effects of bacitracin (BC) and excess Mg2+ On pellet formation and mycelial growth of Streptomyces azureus were studied in a liquid culture using Bennett medium. The addition of Mg2+ at concentrations above 0.2 mM to the medium resulted in the promotion of pellet formation, a distinct inhibition of growth and decrease in cell mass. BC changed the growth type of mycelia from the compact pellet-type to the dispersed-type, and stimulated mycelial growth after a long lag period, accompanied by an increase in cell mass. In the presence of less than 0.2 mM Mg2+, BC completely inhibited mycelial growth. Ca2+ showed an effect similar to Mg2+. EDTA inhibited pellet formation, but never stimulated growth. Furthermore, EDTA suppressed the growth inhibitory and stimulatory actions of BC. From these results, we speculated that : (i) excess Mg2+ induced pellet formation; (ii) BC, similar to EDTA chelated with excess Mg2+ in the medium, leading to the inhibition of pellet formation; and (iii) BC-induced stimulation of growth might be due to its chelating and antimicrobial activities. It was also said that the growth inhibitory action of BC on S. azureus was antagonized by excess Mg2+ or Ca2+ and EDTA.