A monocytic cell line, termed Mono Mac, was established from peripheral blood of a patient with monoblastic leukemia. Two clones, designated Mono Mac I and Mono Mac 6, were isolated and both were assigned to the monocyte lineage on the basis of morphological, cytochemical and immunological criteria. Most importantly, the clones express NaF-sensitive non-specific-esterase, produce reactive oxygen and stain with MAb My4. Mono Mac 6, in addition, constitutively exhibits phagocytosis of antibody-coated erythrocytes in 80% of the cells and reacts with a panel of MAbs that are specific for mature monocytes, i.e., M42, LeuM3, 63D3, Mo2 and UCHMI. By contrast, the monoblastic cell lines U937 and THP-I are negative for all these markers. Only expression of My4 could be detected after differentiation induced by interferon-γ (IFN-γ). Similar treatment of Mono Mac I, however, resulted in staining with all the monocyte-specific MAbs mentioned above, while IFN-γ treatment of Mono Mac 6 enhanced antigen expression. In addition, the cells showed an increased frequency of multinucleated cells with a rise from 4.8% to 21.9%. Mono Mac 6 appears to be the only one of the cell lines studied to constitutively express phenotypic and functional features of mature monocytes.