Coagulase-negative and deoxyribonuclease-negative mutants were isolated from Staphylococcus aureus BB by treatment with N-methyl-N'-nitro-N-nitrosoguanidine. Comparison of virulence (50% lethal dose) to mice of these six mutant strains and S. aureus BB was determined by both intravenous and intraperitoneal routes. The ratios of the 50% lethal dose of coagulase-negative mutants to that of the parental strain S. aureus BB ranged from 201 to 403 for intravenous infection and 30.7 to 52.7 for intraperitoneal infection. The virulence of deoxyribonuclease-negative mutants was essentially the same as that of S. aureus BB. When mice were immunized subcutaneously with live S. aureus BB or its deoxyribonuclease-negative mutants, the resulting protection against the intravenous challenge of S. aureus BB was remarkable. The ratios of the 50% lethal dose for the mice that were immunized by these strains to that for the untreated mice extended from 40.1 to 60.6 for intravenous infection and 6.61 to 11.5 for the intraperitoneal route. However, no effect against S. aureus BB challenge was shown in the mice that were immunized with coagulase-negative mutants.