Homogenates of human cancer tissues were assayed for chemotactic activity in modified Boyden chambers utilizing polycarbonate filters with an 8μ pore size. Peritoneal exudate cells (PEC) of guinea pigs were used as indicator cells in the assays and the chemotactic responses of both eosinophilic and neutrophilic leukocytes were recorded. The chemotactic responsiveness of leukocytes to the majority of homogenates was significantly less than that observed with the negative controls. It is suggested that the anti-chemotactic activity observed in vitro may operate in vivo to abrogate inflammatory responses and tumor rejection