We have investigated the contribution of immunoglobulin to the liposome-induced activation of complement in human serum. Liposomes containing the negatively charged phospholipids cardiolipin, phosphatidylglycerol or phosphatidylinositol, in addition to phosphatidylcholine and cholesterol, were used to activate complement in a whole serum system. The contribution of immunoglobulin was studied by comparing normal human serum (NHS) to serum depleted of IgG and IgM (DDS). Using hemolytic assays of complement function, greater concentrations of phospholipids were required to activate complement in the absence of immunoglobulins. Activation of the classical pathway was confirmed using a C1q ELISA which showed that activation was dependent on the presence of C1q and confirmed that greater concentrations of phospholipids were required to activate complement in the absence of immunoglobulins. Complement activation was also assessed using crossed immunoelectrophoresis of C3 activation fragments. Using immunoblot analysis, iC3b was detected on the surface of liposomes exposed to NHS or DDS. These studies demonstrate that when liposomes, containing anionic phospholipids at an equivalent charge to cardiolipin 20 mol%, are exposed to immunoglobulin depleted serum they become opsonized by complement proteins.