An EBV-transformed lymphoblastoid B cell clone (A12) derived from peripheral blood lymphocytes of an HIV-1-infected individual is described. The immunoglobulin isotype produced by this clone was IgM, and Southern blot analysis of immunoglobulin gene rearrangement showed a monoclonal pattern. The A12 monoclonal antibody was specific for the p24 product of the HIV-1 gag gene. This clone is now in continuous culture for more than 8 months and no changes in its biologic properties have been observed.