Serologic tests for the diagnosis of blastomycosis have had inadequate sensitivity, but results are improved with enzyme immunoassay (EIA) with A-antigen of Blastomyces dermatitidis. The Premier Blastomyces EIA kit (Meridian Diagnostics Inc, Cincinnati, Ohio) detected antibody against the B dermatitidis A-antigen in sera from 104 of 125 patients (83%) with culture-proven blastomycosis. Semiquantitative index values (positivity multiplied by the dilution) ranged from 1.0 to 423.3; the degree of positivity correlated with disease activity. Immunodiffusion bands were infrequentiy detected, with positive specimens from only 19 (21%) of 92 patients with blastomycosis. In 8 of 24 patients with histoplasmosis, 24 of 51 specimens (47%) were positive for Blastomyces antibody by EIA; index values were lower, with a mean of 3.8 versus 24.3 for sera from patients with blastomycosis. Only 1 of 13 patients with other fungal infections had positive results with EIA. This EIA system provided better sensitivity than previously available commercial systems for serodiagnosis of blastomycosis. © 1995 Southern Medical Association.