This study reports on the earliest stages of mononuclear phagocyte differentiation. A crucial question in this developmental process is whether mature macrophage (Mϕ) heterogeneity is already appointed at the precursor cell level. For this purpose, we produced clonal populations of mononuclear phagocytes from bone marrow culture by somatic cell hybridization with two hypoxanthine, aminopterin, thymidine‐sensitive myeloid cell lines. A panel of 22 stable hybrids was obtained from these fusions. Differentiation stage analysis of the hybrids indicated that all cell lines had immature mononuclear phagocyte characteristics. The hybrids exhibited typical myeloid morphology and mainly nonadherent growth. Mature Mϕ features, such as expression of the cell surface antigens Mac‐1, Mac‐2 and F4/80, phagocytosis of latex beads, and expression of nonspecific esterase and acid phosphatase activity, were virtually absent. The immature Mϕ markers Thy‐1, MIV 25 and MIV 52, on the other hand, were readily expressed, although heterogeneity was observed among different hybrid cell lines. We then analyzed the differentiation potential of seven hybrids by culture of the cells in the presence of post‐lipopolysaccharide serum supplemented with interferon‐γ and found that the expression of mature Mϕ characteristics was induced. However, the various hybrids showed divergent patterns of mature Mϕ marker induction. ROC2 cells, for instance, showed extensive morphological and phenotypical differentiation without concomitant induction of phagocytosis. In contrast, W1C4 cells showed significant induction of phagocytosis without simultaneous increase of phosphatase and esterase activity. R1C1 cells were unique in the strong induction of Ia antigen expression. Together, our data indicate that (a) early Mϕ differentiation stages can be rescued by somatic cell hybridization, and that (b) the obtained cell lines are able to mature according to divergent differentiation programs.