This paper examines the interaction in serum free media of different particles with normal or stimulated mouse macrophages cultivated in the presence or in the absence of serum. Thioglycollate-stimulated macrophages cultivated for 7 weeks with or without serum phagocytized antibody-coated erythrocytes as well as a variety of particles that tested for “non-immunologic” recognition. After different periods of cultivation, however, depending on the media used, these macrophages failed to interact with erythrocytes coated with amboceptor and complement (EAC). Normal macrophages bound EAC after 2 weeks in culture with serum but failed to do so when serum was omitted from the culture medium. It is concluded that receptors for complement in cultivated macrophages are more labile and require serum for their manufacture or maintenance, in contrast to the apparent stability of receptors for immunoglobulin and non-immunologic phagocytosis.