In this study, a technique for the evaluation of circulating leukocyte phagocytosis has been adapted to mouse peritoneal macrophages. This quantitative determination was performed by the spectrophotometric measurement of the reduced nitroblue tetrazolium (NBT) fixed on bacteria or latex spherules. Capacity of phagocytosis was thus correlated with the intensity of the NBT intracellular reduction by macrophages. This method is technically simple and requires no expensive materials. The phagocytosis of latex did not significantly differ, neither in the presence of autologous or heterologous serum (X +/- G = 0.100 +/- 0.014/2.5 X 10(6) cells) nor in the absence of serum (X +/- G = 0.088 +/- 0.007/2.5 X 10(6) cells). The use of macrophages suspended in the culture medium highly decreased the phagocytic activity (X +/- G = 0.021 +/- 0.002/2.5 X 10(6) cells) and confirmed thus the role played by the support in endocytosis. A specific antiserum weakly enhanced the phagocytic process for Escherichia coli. The mean values with and without serum were 0.065 +/- 0.005/2.5 X 10(6) cells and 0.050 +/- 0.005, respectively. Heating of the serum (56 degrees C, 30 min) and inhibition of both complement activation pathways by EDTA showed that complement plays the major role in this weak enhancement of bacterial phagocytosis by peritoneal mouse macrophages. Bacterial phagocytic stimulation of macrophages was not induced by addition of CA+++ or Mg++ into the culture medium.