Fulvocin C is a bacteriocin from Myxococcus fulvus Mx f16. It has a molecular weight of 4672 and is one of the smallest bacteriocins known. Four disulfide bonds give the molecule a tight structure, so that its native form was not attacked by chymotrypsin or pronase. Fulvocin C was stable in various organic solvents and could tolerate 80 degrees C in aqueous solution without loss of activity. The killing effect of fulvocin C was observed only at concentrations higher than 0.25 mumol/1. Macromolecular synthesis (DNA, RNA, protein) was affected very gradually. Viability in growing cultures decreased slowly from 100 to 25% during one generation (8 h). Cell division was affected early. After one generation v-shaped cell pairs had accumulated in the culture. Electron microscopic pictures revealed extended membrane systems connected with the inner membrane. The most striking effect was that often the outer membranes of neighbouring cells seemed to have fused laterally. With further incubation many cells lost their rod shape and empty bags became predominant.