We have investigated the expression of the common acute lymphoblastic leukemia antigen (cALLA) by nonlymphoid cells, using a new murine monoclonal antibody (designated 24.1), specific for cALLA. This antibody completely blocks the binding of monoclonal anti-cALLA antibody J-5. Furthermore, antibody 24.1 binds to cALLA with greater affinity and induces a greater degree of antigenic modulation than antibody J-5. With antibody 24.1, we have demonstrated that normal cultured marrow and skin fibroblasts and mature granulocytes express cALLA. By immune precipitation and polyacrylamide gel electrophoresis, cALLA from fibroblasts has a slightly lower molecular weight and cALLA from granulocytes a slightly higher molecular weight than cALLA from cultured human leukemic cell lines. Quantitative studies indicate, however, that cALLA expression is approximately 3-fold and 20-fold lower, respectively, on normal granulocytes and skin fibroblasts than on NALM-6 cells. cALLA may be expressed by cells with widespread distribution in multiple organ systems. These findings emphasize that differentiation markers that appear to be tumor or tissue specific may be found on cells of diverse origin.