Prevention of ropy bread caused by mucoid variants of certain bacilli presents a major problem for developing countries where cost of preservatives is prohibitive. Control of ropiness may be achieved by using propionic acid-producing bacteria in mixed culture with leavening yeasts. Therefore, interaction studies between Propionibacterium freudenreichii subsp. shermanii, Bacillus pumilus and Saccharomyces cerevisiae were conducted in a chemically defined medium to test the relevance of such an approach. Growth of vegetative cells and germination of spores of B. pumilus were inhibited in media preincubated with P. shermanii at 30 degrees C for 13 h. Inhibition was bacteriostatic for the first 6 h of incubation, becoming bactericidal between 6 and 12 h. Inhibition of B. pumilus spore germination was greater than inhibition of growth of vegetative cells of the bacterium. Culturing of either P. shermanii with S. cerevisiae or B. pumilus with S. cerevisiae did not produce inhibitory effects on any of the organisms. Inhibition of B. pumilus by P. shermanii may be useful for prevention of ropiness in bread prepared by the sponge method, involving fermentation of a portion of the dough.