Arthroconidia stripped of their outer, hydrophobic wall layer release an immunoreactive, water-soluble fraction (SCWF), the composition of which is examined in this paper. The immunogenicity of SCWF was determined by its reactivity with anti-Coccidioides immitis complement-fixing antibody and tube precipitin antibody in a standardized immunodiffusion assay as well as reactivity in a lymphocyte proliferation assay. SCWF was shown to be more immunoreactive for immune lymphocytes than selected cytosol and culture supernatant fractions of C. immitis. Rabbit antisera raised against SCWF were used in immunoelectron-microscopic and immunofluorescence studies to confirm that the immunoreactive components of SCWF are primarily associated with the inner conidial wall. The antigenic composition of the conidial wall fraction was characterized by advancing-line immunoelectrophoresis with the previously established coccidioidin reference system. Protein composition was characterized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Chromatographic fractionation of SCWF was performed on a Sephacryl S-300 preparative column. Selected fractions which showed significantly higher immunoreactivity and less complex antigenic composition than whole SCWF were characterized. Two heat-sensitive antigens of Sephacryl fraction 3a were identified. One or both of these antigens may correspond to the complement-fixing antigen, which is potentially important as an immunodiagnostic antigen. Fraction 4 contained a previously described wall-associated antigen (AgCS) which may also be of immunodiagnostic value. We conclude that the conidial envelope is a reservoir of immunoreactive macromolecules which may play significant roles in early stages of infection.