The prevalence of Fusarium solani reactivity in atopic individuals with symptoms of mold allergy was assessed with skin test and RAST. In addition, F. solani preparations were evaluated for antigenic/allergenic activity. Atopic individuals tested, 24.5% (n = 69), had positive skin reactions to a phosphate-buffered saline extract of F. solani, and these responses were statistically correlated with RAST results. Immunoelectrophoretic techniques demonstrated that this extract was antigenic in rabbits and allergenic in man. Gel filtration of F. solani extract on a Bio-Gel A 0.5 m column demonstrated three peaks of ultraviolet-absorbing material. The column eluate with the greatest RAST inhibition activity was associated with a protein peak having a molecular weight greater than 341 kilodaltons; however, all peaks demonstrated inhibitory activity. These studies suggest that extracts of F. solani contain several allergens that differ in molecular weight and charge.