For the purpose to clarify whether electrostatic contributes to the adhesion of red blood cell to monocyte, first phase of phagocytosis, was studied in vitro in ion deficient environments comparing with that in ionic environments. The experiments were carried out by using mouse ascites macrophage and the fresh and fixed red blood cells of mouse and chicken. Observations revealed that in Hanks' solution, the fixed red blood cells adhered to the monocyte, while fresh red blood cells of both species did not. In isotonic sugar aqueous solutions, however, the fixed red blood cells did not adhere to the monocyte. The test in the varied ionic concentration proved that number percent of monocytes adhering the fixed red blood cells to the total monocytes reduced in proportion to logaritsm of the decrease in ionic strength of the media. Concentration of Ca(++) and Mg(++) and pH of the media gave actually no effect on the red blood cell adhesion to the monocyte. The red blood cells fixed and conditioned with methylation treatment adhered to the monocytes even in ion free sucrose solution, but those conditioned with desamination treatment did not. From these results it is concluded that the ionic strength in the media is a decidive factor for the fixed red blood cell adhesion to the macrophage. Discussion was made on the mechanism of the adhesion from the view point of lyopobic and lyopilic colloid mechanism.