With the use of the monoclonal antibody BA-1, we have identified and characterized a unique molecular complex on the surface of human B lymphocytes. Despite the marked pronase sensitivity of the BA-1 antigen/epitope on the cell surface, repeated attempts to surface or biosynthetically radiolabel the antigen by using standard protein labeling techniques were unsuccessful. The antigen was identified by immunoprecipitation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis after radiolabeling carbohydrate residues. Surface labeling of galactosyl residues by the neuraminidase/galactose oxidase/3HNaBH4 method revealed a three chain, nondisulfide-linked glycoprotein complex of 45, 55, and 65 kilodaltons (kDa), which we have designated gp45/55/65. Under nonreducing gel conditions there were shifts in the mobility of the 55- and 65-kDa components, consistent with the presence of intrachain disulfide linkages. Surface labeling of sialic acid residues by the sodium periodate/3HNaBH4 method revealed the presence of sialic acid residues on the 45- and 55-kDa components. Two-dimensional gel analysis indicated that the 55-kDa component had an acidic pI of approximately 4.0 and considerable charge heterogeneity, most likely attributable to sialic acid. In contrast, the 45-kDa component had a neutral pI of approximately 7.5. Despite the difficulty in radiolabeling amino acids in gp45/55/65, digestion with V8 protease and pronase clearly demonstrated the presence of protein cores in all three components. Consonant with prior serologic studies with the use of BA-1, we could identify gp45/55/65 on B lymphocytes, B cell precursor acute lymphoblastic leukemias, neutrophils, and eosinophils. This molecular complex has, to our knowledge, not been heretofore described on human B cells and is quite novel in its structure and labeling properties.