In the present paper we describe the production and characterization of a monoclonal antibody (Mab) recognizing HLA-Aw32 + A25 antigens. NS1 murine myeloma cells were fused with splenocytes from a BALB/c mouse immunized with normal human peripheral blood (PB) lymphocytes of phenotype A1, Aw32; B7,B37,Cw-,Cw-;DR2,DRw10. Supernatants were first screened against Cr51-labeled immunizing cells by complement dependent cytotoxicity 51Cr-CDC). Cultures identified as producing cytotoxic antibodies were subcultured and the supernatants tested against a selected panel of HLA typed cells by the NIH microcytotoxicity method. One culture producing antibody reacting with an HLA polymorphism was detected. This hybrid, designated CATA 1, was cloned twice by limiting dilution and obtained in ascitic form. Specificity of CATA 1 Mab was evaluated against a panel of 120 PB T cells from normal donors. CATA 1 reacted with cells bearing HLA-A25 or HLA-Aw32 antigens. In addition, a reaction was observed with a cell of phenotype A2,Aw31; B17,Bw49. Isoelectric focusing revealed the monoclonal nature of CATA 1, with immunofixation identifying it as an IgG molecule. Absorption studies have demonstrated that CATA 1 recognizes a common determinant on HLA-A25 and HLA-Aw32. The finding that this Mab recognizes the same CREG as alloantisera against HLA-Aw32 suggests that this antigen has no unique epitopes.