A specific immunodiffusion test for blastomycosis has been developed. The test permitted the detection of approximately 80% of 113 proven cases of blastomycosis. Two diagnostically important precipitins designated A and B were frequently recognized in patients with blastomycosis. Routine use of reference sera containing the A and B precipitins in immunodiffusion tests would permit the specific diagnosis of blastomycosis without the need for parallel tests with coccidioidin and histoplasmin.