YAC-lymphoma inoculation (ip) into syngeneic A/J mice results in a massive in situ proliferation of the lymphoma cells, with non-significant increases in the peritoneal macrophage population, and also in a remarkable and progressive increase in the bone marrow precursor cells forming both macrophage and fibroblastoid colonies in vitro. In the allogeneic situation (BALB/c mice), YAC-lymphoma cells proliferate to a limited extent before tumour regression. These processes (i.e. proliferation and regression) were accompanied by a progressive increase in the peritoneal macrophage population, which declined to normal after tumour regression. Macrophages accumulating at the regression stage exhibited in vitro cytolytic activity towards YAC-lymphoma cells and cytostatic activity towards unrelated cells. No correlation was observed between the limited asynchronous increase in the bone marrow level of precursor cells for in vitro differentiating macrophage and fibroblastoid colonies, on one hand, and macrophage accumulation at the tumour site in BALB/c mice on the other. Conditioned media from in vitro proliferating YAC-lymphoma cells exhibited high colony-stimulating activity for macrophage colony formation in vitro. Bone marrow cells taken from YAC-lymphoma inoculated A/J mice expressed potent tumorogenic potential at early stages after tumour inoculation. At day 2 after tumour inoculation the bone marrow cells were already tumorogenic and concomitantly expressed augmented levels of macrophage and fibroblastoid colony-forming capacity. The results indicate an inter-relationship between tumour growth and mononuclear phagocyte differentiation and proliferation.