BACKGROUND Snyder-Theilen feline sarcoma virus (ST:FeSV)-transduced human fibroblasts differentiate into tissue macrophages(1-9). The ST:FeSV-induced macrophages demonstrate macrophage-mediated cytotoxicity (MTC) and antibody-dependent cellular cytotoxicity (ADCC), including growth modulation of tumor cells in agar (3,4,6). METHODS Here we tested the effects of ST:FeSV-induced macrophages in agar on the following human tumor cell lines: colon adenocarcinoma, prostate adenocarcinoma, breast adenocarcinoma, malignant melanoma, leiomyosarcoma, fibrosarcoma, and fibrous histiocytoma. The tumor cells were co-incubated in agar with ST:FeSV-induced macrophages in the absence or presence of 10% fetal bovine serum (FBS). RESULTS Regardless of serum conditions, the growth of all tumor cells tested was inhibited considerably by the ST:FeSV-induced macrophages. Colon adenocarcinoma cells were the least affected, and fibrosarcoma or fibrous histiocytoma cells were the most sensitive to growth inhibition by the ST:FeSV-induced macrophages. A notable exception was the growth stimulation of breast adenocarcinoma (BT-20; MCF-7), and of prostate adenocarcinoma (TSU-prl) tumor cell lines by the ST:FeSV-induced macrophages in the absence of FBS. CONCLUSIONS The results attest to the potency of secreted proteins that are expressed by ST:FeSVinduced macrophages which can modulate tumor cell growth in agar. The results further indicate that serum is likely to have an impact on the effects of these growth regulatory factors on human tumor cells.