A cell migration inhibitory effect was evidentiated in normal guinea pig serum as compared with heat inactivated serum. Granulocytes when used as target showed a greater sensitivity to this effect than lymphomonocytes or macrophages. The migration inhibitory activity of GPS is abrogated or decreased by using complement destroying agents such as: heating at 56 degrees C for as little as 5 min, absorption on immune complexes in presence of calcium, on zymosan, on Sephadex G-50 or by adding EDTA or heparin to culture medium. The GPS dialysation fractions while exhibiting neither complement haemolytic effect nor migration inhibitory activity when tested alone, restored these functions by recombination. Absorption of GPS on homologous blood cells abrogated the migration inhibitory effect but retained the complement haemolytic function. When GPS absorbed on homologous blood cells mixed 1:5 with heat-inactivated serum (5 min at 56 degrees C), the migration inhibitory activity was regained, suggesting that the complement factors from the first sample were necessary for manifestation for the migration inhibitory activity from the heat-inactivated serum.