Cytogenetic analysis and Feulgen--DNA cytophotometry were performed on murine resident and exudate peritoneal macrophages. A large proportion (14--36 per cent.) of metaphase profiles from cell populations rich in resident macrophages displayed chromosomal aberrations compared with 0--8 per cent. seen in metaphases from cell populations rich in exudate macrophages. The most common aberration observed was the presence of chromosomal gaps. It is suggested that should the resident macrophage represent a later, and perhaps terminal stage of macrophage development, the high frequency of aberrations found may be a result of cell ageing.