Complex polysaccharides and lipopolysaccharides can activate the terminal components of complement by either the classical (antibody, C1, C4, and C2) or alternative complement pathways, but the relative importance of either pathway for terminal component consumption in normal serum is poorly understood. Since classical complement pathway function requires both calcium and magnesium ions, whereas the alternative pathway requires only magnesium ions, selective chelation of calcium ions in serum can be used to block the classical complement pathway while leaving the alternative pathway intact. In these studies, ethyleneglycol-bis-(beta-aminoethyl ether)N, N-tetraacetic acid, a potent chelator or calcium, was used to block the classical complement pathway in normal guinea pig serum. Consumption of the terminal complement components by endotoxin, inulin, and zymosan in such serum was strikingly depressed when compared to serum containing an intact classical complement pathway. These studies demonstrate that in normal serum, both the classical and alternative complement pathways participate in the consumption of the terminal complement components by complex polysaccharides and lipopolysaccharides.