A vaccine consisting of formaldehyde-killed spherules of Coccidioides immitis, previously shown to provide protection against development of lethal coccidioidomycosis in laboratory animals, was evaluated in humans. This double blind "Phase 3" study, conducted during the period 1980 to 1985, involved 2,867 healthy subjects with no history of coccidioidomycosis and negative skin tests. Randomized into vaccine (n = 1,436) or placebo (n = 1,431) groups, the former received three intramuscular injections of 1.75 mg (dry weight) of spherules, the latter received three injections of sterile 0.85% NaCl solution. Compatible clinical presentation with cultural or serologic findings permitted detection of coccidioidomycosis. Of those receiving vaccine, nine developed coccidioidomycosis and nine additional were suspected of having the disease. Of the group receiving placebo, 12 developed coccidioidomycosis, and 13 additional were suspected of having the disease. All cases and suspected cases were mild. Under the conditions of this study, a slight but statistically insignificant reduction in the incidence of coccidioidomycosis was noted in the vaccinated group compared with that in the placebo group, and there was no discernible difference in severity of disease between these groups.