, D. C. SAVAGE, AND LEIGHTON N. L. KONG. Delayed dermal hypersensitivity in mice to spherule and mycelial extracts of Coccidioides immitis. J. Bacteriol. 91:876-883. 1966.-A delayed hypersensitivity reaction to spherule and mycelial extracts of Coccidioides immitis was elicited in the footpads of mice vaccinated with killed spherules. Emulsification of the spherules with Freund's adjuvants was unnecessary, but a high concentration of antigen was required to elicit the reaction. Injection of the extracts produced, initially, a swelling which subsided within 4 hr, and then induration, which began at 6 to 8 hr and reached a maximum at 24 hr. The time course of the reaction corresponded to that of the tuberculin reaction in BCG-vaccinated mice. The histological response to coccidioidal extracts was characterized by the early infiltration of both polymorphonuclear and mononuclear cells, and the subsequent predominance of mononuclear cells at 24 to 48 hr. By 72 hr, the mononuclear cells comprised >90% of the cellular infiltrate. Animals infected intranasally with arthrospores (1 to 5 LD15) reacted negatively before and during the crisis period; thereafter (by 28 to 31 days after infection), up to 50% of the survivors showed a delayed reaction.