Fibronectin (Fn) synergizes with macrophages (M phi) in inducing cytostasis and cytotoxicity of neoplastic cells in culture. Since heparin enhances Fn's opsonic activity in many systems, we investigated its effect on Fn-macrophage synergy in cytostasis. MCG-T14 (a spontaneous mouse mammary adenocarcinoma) cells (4 X 10(4] were added to wells both with and without C. parvum activated M phi monolayers. To these cultures were added increasing concentrations of Fn with or without heparin. Fn synergizes with both M phi S and heparin in inhibiting tumor cell growth. The combined cytostatic effect of Fn, heparin and M phi S is more than additive. In other experiments, MCG-T14 cells were pre-incubated for 2 hr with Fn, washed free of Fn, and treated as above. The results of these experiments were similar to coculture experiments, but the effect of heparin was even more pronounced. Dermatan sulfate and hyaluronic acid had a variable effect on Fn and Fn-macrophage induced cytostasis.