Bacilysin-negative(bac-) strain NG79 was found to be oligosporogenous. When compared with the parental strain, it was 200-300 times less resistant to heat, chloroform, and lysozyme treatments, and the spores contained considerably less dipicolinate. When NG79 was transduced, the oligosporogenous phenotype was found to be cured in all the transductants tested. External addition of bacilysin to the cultures of this strain markedly improved each measure of spore quality. The time of its addition determined the extent of acquired resistance, the optimum being 4-7 h after inoculation. This suggested that bacilysin might influence sporulation prior to stage I. Bacilysin activity completely disappeared from the extracellular fluid of aged cultures. It was demonstrated that the dipeptide can be cleaved by the alkaline serine protease that is produced by Bacillus subtilis.