Skin prick test activity and antigenicity of extracts of in vitro growth of the Basidiomycete Pleurotus ostreatus (PO) were compared to extracts of spores from PO growing in the wild. Patients demonstrated significant differences in skin test reactivity to the PO extracts. Some reacted only to in vitro growth extracts, others only to the spore extracts and 1 patient to all extracts. Further studies analyzed antigens present in all extracts with rabbit antisera to PO. Common as well as unique antigens were present in the spore extracts as compared to those from in vitro preparations. The fact that spores contain unique antigens suggests that basidiospores may be the best source of relevant allergens for clinical studies.