Other investigators have reported activation of the alternative complement (C) pathway in homologous and heterologous serum by a variety of human lymphoblastoid B cell lines. Their ability to activate the C was associated with Epstein Barr virus transformation of the cells. We report that some human lymphoblastoid T cell lines, lacking EBV-DNA in their genome, do activate the human alternative C pathway with no participation of immunoglobulin or specific antibodies. Nevertheless, activation measured by C3 deposition on the cell surface was weaker than with the B cell lines so far studied; in relation to the RAJl cells, C3 deposition on JURKAT, MOLT 4, HSB2, and 1301 was, respectively, 68, 38, 28, and 19%. Furthermore, C3 deposition on JURKAT T cell line requires D, whereas RAJl cells provide a proteolytic activity able to mimic D, and which, unlike D, can be controlled by serum protease inhibitors. Although the ability to activate the AP seems to be a largely shared property of the human lymphoblastoid B and T cell lines, the situation was strikingly different with normal and mitogen-stimulated lymphocytes or with acute leukemic cells, which led to a negligible AP-dependent C3 deposition compared with the level observed with the lymphoblastoid cell lines. Membrane sialic acid content was determined for every cell tested and revealed no relationship with their ability to activate the AP. The two EBV+ B cell lines tested led to a comparable AP activation regardless of the presence of the C3b receptor.