Polystyrene and phenylated polyacrolein microspheres of different diameters, as well as modified cellulose microspheres with different surface charges, were prepared in order to study the size and surface charge effect on their phagocytosis by mouse peritoneal macrophages. It was found that the maximal phagocytosis of polystyrene and phenylated polyacrolein microspheres took place when their size was in the range 1.0-2.0 microns. Microspheres with hydrophobic surfaces were more readily phagocytosed than those with hydrophilic surfaces. There was no significant difference in phagocytosis between cationic and the anionic surfaces when compared at a zeta potential of the same absolute value. The least phagocytosis was observed for cellulose microspheres with non-ionic hydrophilic surfaces. Addition of fetal calf serum to the culture medium resulted in decrease in phagocytosis for all microspheres.