Frentizole, 1-(6-methoxy-2-benzothiazolyl)-3-phenyl urea, a new immunosuppressive agent, and azathioprine were administered subcutaneously at predetermined immunosuppressive dose levels of azathioprine and up to 50 times an immunosuppressive dose level of Frentizole. After 10 days of treatment at these dose levels, the experimental groups were inoculated intraperitoneally with Pseudomonas aeruginosa or herpes simplex virus, inoculated intraveneously with Candida albicans, or infected by aerosol with Ann Arbor influenza virus. The results of these series of experiments indicate that Frentizole, even at super immunosuppressive doses, does not predispose the hose (mice) to Pseudomonas aeruginosa, Candida albicans, herpes simplex virus, or Ann Arbor influenza virus.