A synergistic effect on mouse mortality was demonstrated in combined infection of mice with Candida albicans and a Staphylococcus aureus strain isolated from a patient with toxic shock syndrome. Mice exhibited high resistance when inoculated intraperitoneally by either pathogen alone. Dual infection with the two organisms together, however, at doses which separately caused no animal deaths, resulted in 100% mortality. This synergistic effect could not be reproduced when either of the agents was heat inactivated.