In the present study, we have analyzed the effects of the anti-FAM mAb 5C6, on endogenous NK cell and lymphokine-activated killer (LAK) cell activity of swine. We report that the anti-FAM mAb 5C6 binds to fresh and cultured lymphocytes with spontaneous cytotoxic activity from swine peripheral blood. In conjunction, the anti-FAM mAb 5C6 inhibits swine cytolysis against K562 target cells. Inhibition was found to be mediated by blocking of conjugate formation. The level of spontaneous killing and mAb inhibition was correlated with the number of swine cells staining positive with the mAb 5C6 as assessed by flow cytometry.