We examined the isoenzyme patterns of alpha and beta naphtyl acetate esterase and the IL6 production of two macrophage cell lines, which were cloned from a single fusion of macrophage tumor cells and spleen adherent cells. These clones were phenotypically indistinguishable but differ functionally as line 59 presents antigen to Th 1 lymphocytes while line 63 induces suppressor T lymphocytes. Cell extracts of these lines exhibit different isoenzyme patterns of alpha and beta naphtyl acetate esterase at both pH 7.5 and 5.8 but do not differ significantly in the level of produced IL6. Treatment with nitrogranulogen (NG), a derivative of cyclophosphamide, changes the isoenzyme pattern in line 59 and decreases severalfold IL6 production, while in similarly treated line 63 cells isoenzyme pattern remains unaffected but the production of IL6 is significantly increases. We assume that the observed differences between these two cell lines are due to distinct intracellular translation of the membrane signal delivered by NG. The different behaviour of these two parameters can thus be used as a useful tool to further delineate different macrophage subpopulations. We regard it likely that nonspecific esterases may play a role in intracellular processing or trafficking of antigen.