We have investigated the production of C3, C4, and factor B complement components in primary cultures of murine astrocytes and in clonal cell lines belonging to the astrocytic lineage by immunoprecipitation of secreted labeled polypeptides. Although C4 has not been detected, C3 appeared to be constitutively synthesized both by two transformed astroblastic cell lines and by astrocytes in primary cultures. In contrast, factor B was only secreted upon lipopolysaccharide stimulation both in astroglial primary cultures and in an immortalized astrocytic cell line. The eventual physiologic relevance of an endogenous brain production of components of the alternative pathway of complement activation is discussed.