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Antibodies directed against the common acute lymphoid leukemia antigen (CALLA) were obtained from 2 hybridomas: J5 (Schlossman, mice sensitized with patient ALL cells), and Vil-A1 (Knapp, sensitization with the Reh cell line). The percentage of lymphoid cells reacting with these 2 monoclonal antibodies were compared. Antibody dilution curves indicated that the dilutions used yielded maximum percentages of positive cells. The percentage of CALLA-positive cells with the J5 antibody was significantly (p less than 0.001) higher than that found with the Vil-A1 antibody in 16 non-neoplastic inflammatory tonsils and in 13 non-Hodgkin lymphoma and chronic lymphatic leukemia lymph-nodes (p less than 0.05). In contrast, the difference between CALLA positive cells with J5 and Vil-A1 was not significant (p greater than 0.5) in 19 acute lymphoid leukemias. The difference between the ALL-cells, presumably pre-B, and the B-cells from the non-ALL subjects was also statistically significant (p less than 0.01). The results suggest that the two hybridomas form antibodies against different CALLA epitopes. Vil-A1 seems somewhat more specific for ALL than J5.