We have previously described a monoclonal antibody, referred to as 38.13, reacting with most Burkitt's lymphoma (BL) derived lines, either containing the Epstein-Barr virus (EBV) or not, but not reacting with EBV-positive lymphoblastoid cell lines (LCL). (PNAS 78: 6485, 1981; Int. J. Cancer 29: 653, 1982). The antibody reacted with 41 BL lines out of 57 tested so far. The target antigen. BLA, was shown to be a neutral glycolipid, identified as globotriaosylceramide (Gal alpha 1----4 Gal beta 1----4 Glc beta 1----1 Ceramide). This substance is known as Pk blood group antigen, a normal intermediate in the P substance synthesis. Pk antigen is detectable only on the erythrocytes of very rare individuals with the Pk phenotype. We showed that 38.13 antibody stained erythrocytes and fibroblasts from 2 Pk subjects, whereas it was unreactive with erythrocytes and fibroblasts of all the other individuals tested. The accumulation of globotriaosylceramide on BL cells could reflect some functional disturbance of the enzymes in the P substance synthesis pathway.