Fab fragments of a monoclonal antibody directed against p37, a protein associated with the surface of FS9 mouse sarcoma cells, were previously found to inhibit the highly invasive behaviour of FS9 cells in vitro. We show that p37 originates from Mycoplasma hyorhinis. Infecting various cell lines with the mycoplasma consistently increased their invasiveness when confronted with chicken heart fibroblasts using Abercrombie's confronted explant technique. Conversely, removing the mycoplasmas or blocking p37 with specific Fab fragments reduced invasiveness. Analysis of individual cell collisions using time-lapse filming showed that the addition of Fab fragments to cells infected with M. hyorhinis greatly increased the level of contact inhibition. The antibody also reduced the invasiveness of transformed cells that did not express the p37 antigen. Hence, a cellular protein or proteins that are structurally related to p37 apparently influence invasive behavior.