The binding of polymorphonuclear leucocytes (PMN) to the glomeruli of rats in which glomerulonephritis had been induced by bovine serum albumin, suggested a biological activity of the immune complexes localized in the glomeruli. To evaluate divalent cations or serum factors which may be involved, a two-step method for PMN binding was introduced. PMN were suspended in serum-free media and incubated with cryostat kidney sections pretreated with divalent cation-chelated or fresh sera. An appropriate concentration of Mg++ was found to be essential for maximal PMN binding; however, Ca++ may represent a partial substitute for Mg++. Immune complexes localized in the glomeruli fix and activate the complement system via both the classical and the alternative pathways.