Summary: Two coccidioidins prepared by different methods were studied for skin test activity by comparison in guinea pigs, rabbits and human beings. There were 144 test comparisons in two series of guinea pigs, 22 in rabbits and 52 in humans. A single intraperitoneal injection of the coccidioidins induced delayed hypersensitivity in guinea pigs. An enhanced hypersensitivity as measured by skin test response was obtained by incorporation of the coccidioidins in Freund's complete adjuvant. Both coccidioidins were effective in eliciting skin responses in all species tested. When the test coccidioidins were diluted to have equal nitrogen content, their skin test reactivities in rabbits and human beings were the same. A method for standardization of coccidioidins is suggested.