In vivo, circulating blood monocytes (Mo) migrate into the various tissues where they undergo terminal maturation into macrophages (M phi) with morphological and sometimes functional properties that are characteristic for the tissue in which they reside. This tissue-specific M phi heterogeneity results from the immediate microenvironment, but may also originate from genetically distinct Mo subpopulations. The in vitro transformation of Mo to M phi is thought to reflect the events of the in vivo maturation and thus is widely used as a model to analyse M phi development. To study the heterogeneity within the mononuclear phagocyte system, we have investigated the phenotypic characterisation of mature tissue M phi, blood Mo and Mo-derived M phi cultured in medium with either GM-CSF, M-CSF or a combination of both cytokines. Tissue peritoneal and alveolar M phi showed different antigenic specificities, particularly concerning the transferrin receptor and CD68 and CD14 antigens. M-CSF-derived M phi when compared to the other M ø populations also exhibited a significantly increased expression of transferrin receptor and CD68 antigen. In contrast, GM-CSF treated cells which exhibited a better long term survival, showed notably more positivity for CD11b and CD32 antigens. These results show that the phenotypic heterogeneity of the different M phi populations is limited and appears to result from discrepancies in the differentiation and/or activation of the cells. The location of the CD68 antigen, which is generally considered to be an intracellular protein, was investigated at the ultrastructural level and found to be exclusively situated at the outer cell membrane.