We developed a method for avoiding contamination by fibroblasts when cultures of peritoneal cells are initiated. Macrophages were identified by immunogold detection [light microscope, transmission (TEM) and scanning (SEM) electron microscopes] of membrane antigens (Mac-1+, Thy-1,2-), non-specific esterase activity and ultrastructural features (TEM). As compared with controls, the yield of peritoneal macrophages was 2- and 12-fold higher, respectively, in acutely and chronically infected mice. In all, 30 "chronic", 18 "acute" and 18 control cultures were followed up. At a given cell-density seeding, the decline of control, "acute" and "chronic" cultures starts at about day 10, 15, and 27, respectively. In "chronic" cultures only, fibroblast-like cells appear from day 6 onwards; their number increases with time. Cells showing characters intermediary between macrophages and fibroblasts were observed. We suggest that fibroblast-like cells result from the in vitro transdifferentiation of a limited number of in vivo committed macrophages.