Purpose of work Undecylprodigiosin has antimicrobial, immunosuppressive and anticancer properties. Pure cultures of Streptomyces coelicolor produce only low concentrations of undecylprodigiosin. We have therefore sought to increase this by interacting the cells with a bacterium. When interacted with live or heat-killed cells of Bacillus subtilis, Streptomyces coelicolor A3 (2), increased undecylprodigiosin production in shake-flasks and in a 2 1 bioreactor. An inoculum of 2.5% (v/v) B. subtilis containing 10(8) cells/ml added at the beginning of the S. coelicolor culture gave the best results for undecylprodigiosin production. In the shake-flasks, the increase was 175-211% and in the bioreactor, 256% in comparison to the pure cultures of S. coelicolor.