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Diagnosis of coccidioidomycosis largely depends on serologic tests. In this investigation, the enzyme-linked immunosorbent assay (ELISA) was used to detect patient immunoglobulin M (IgM) precipitin antibody binding to a 120-kilodalton (kDa) fraction previously isolated from an alkali-soluble, water-soluble extract of the arthroconidial wall and mycelial culture filtrate plus toluene lysate of Coccidioides immitis. Results of the serologic response to this tube precipitin antigen (TP-Ag) in the ELISA correlated well with results of immunodiffusion assays of 30 serum samples from patients. Immunoelectron microscopic examinations of arthroconidia and spherules were performed with patient IgM precipitin antibodies isolated from sera eluted over a solid-phase immunosorbent column containing the purified 120-kDa TP-Ag. The antibody probe located the 120-kDa TP-Ag on the walls of in vitro-grown arthroconidia and spherules. Pronase digestion and heating (100 degrees C, 5 min) had no apparent effect on the activity of the 120-kDa TP-Ag, while periodate oxidation resulted in total loss of its immunodiffusion-TP activity. Analysis of the carbohydrate composition of the TP-Ag revealed xylose, 3-O-methylmannose (3-O-MM), mannose, galactose, and glucose. Competitive inhibition ELISAs were used to demonstrate that 3-O-MM is largely responsible for the reactivity of IgM precipitin antibodies with the 120-kDa TP-Ag. Synthetic 3-O-MM may be a useful probe for detection of anti-Coccidioides precipitin antibodies in the ELISA.