Peripheral blood monocyte chemotaxis was evaluated in 15 patients with hypernephroma, 10 of whom were evaluated preoperatively and postoperatively. Five patients had a significantly reduced response preoperatively that returned to normal levels after radical nephrectomy. Animal experiments confirm the presence of a low-molecular weight (6,000--10,000 daltons) inhibitor, which is heat labile and functions at the macrophage level. Preliminary experiments indicate inhibitors of macrophage chemotaxis can be extracted from renal cell carcinomas. The importance of these inhibitors in modulating immune surveillance is discussed.