Pemphigus foliaceus (PF) antigen is a transmembrane desmosomal glycoprotein (desmoglein I), part of which is located on the keratinocyte surface. Previous studies have shown that after trypsinization of viable human epidermis, this antigen is no longer detected on the surface of detached keratinocytes. It was not known, however, if this loss of antigenic activity was due to destruction, internalization, or cleavage of the antigen itself. In the present study we investigated the fate of the PF antigen after trypsinization of viable human skin. By using Concanavalin-A agarose affinity chromatography, we could partially purify an antigenic glycoprotein fraction that was released by trypsinization into the medium. This antigenic fraction was radiolabeled and tested by immunoprecipitation using sera from endemic pemphigus foliaceus or fogo selvagem (FS), non-endemic pemphigus foliaceus (NEPF), pemphigus vulgaris (PV), and bullous pemphigoid (BP) patients, and sera from normal subjects as controls. Immunoprecipitated labeled proteins were analyzed by SDS-PAGE and autoradiography. All FS sera (20 of 20 FS and five of five NEPF) and 46% of the PV sera (six of 13) immunoprecipitated a band of 45-kD molecular weight. Sera from FS patients in prolonged clinical and serological remission (seven of 10), sera from BP patients (five of five), and sera from normal donors (nine of nine) did not precipitate this 45-kD band. This study showed that a fragment of the PF antigen is released by trypsinization of human skin as a soluble immunoreactive glycopeptide of 45-kD molecular weight. Additionally, this procedure has generated sufficient quantities of the PF antigen for further biochemical characterization.