The virulence of the coagulase-deficient mutant BB-Cgl- 1301 (50% lethal dose [LD50] for mice by the intravenous route) was compared with that of its parental strain, Staphylococcus aureus BB. The BB strain produced free coagulase of serotype I, whereas the mutant 1301 did not. Mice were infected with strain 1301, alone or in combination with a highly purified coagulase type I or type II solution, or with concentrated culture filtrates of parent strain BB or mutant strain 1301. The ratios of the LD50S of 1301 and its combinations to that of BB ranged from 34.9 to 461. Combining strain 1301 with a concentrated culture filtrate of BB (BB-CF2.5) was the most effective for enhancement of its virulence. When mice were infected with a combination of strain 1301 and BB-CF2.5, the LD50 of strain 1301 (1.72 mg of cells [wet weight]) was decreased to 0.13 mg (1.3 x 10(8) CFU). This LD50 yielded the smallest ratio, 34.9, as compared with the LD50 of BB (0.00373 mg). In contrast, when the mice subcutaneously immunized with strain 1301 and BB-CF50 were intravenously challenged by strain BB, the LD50 for the immunized mice was 17.4 times the LD50 for the unimmunized control mice (0.0429 mg as compared with 0.00246 mg), indicating that combination was the most effective for enhancement of mouse immunization with strain 1301. However, combining strain 1301 with the highly purified sample of coagulase increased neither the virulence nor the immunizing power of mutant strain 1301.