Human glomerular epithelial cells (GEC) contain CD59, decay-accelerating factor, and membrane cofactor protein. Crry is the rodent analogue to the latter two proteins. We have previously shown that the nephritogenic Ab of passive Heymann nephritis, anti-Fx1A, impairs C regulation in rat GEC. Here we examined rat GEC C regulation. 125I-labeled GEC membrane proteins were immunoprecipitated with anti-Crry, anti-CD59, or anti-Fx1A. Crry and CD59 were present in GEC. Anti-Fx1A reacted with both Crry and CD59 from GEC, as well as with purified rCrry and CD59. The alternative C pathway was studied by incubating GEC in rat serum in Mg(++)-EGTA buffer. To inhibit the function of the C regulators, anti-Crry or anti-CD59 Ab were added to GEC. Inhibition of CD59 function alone had no effect on C regulation, whereas inhibition of Crry led to significant cytotoxicity from alternative pathway activation. Under conditions in which Crry was inactive, inhibition of CD59 further enhanced cytotoxicity. When the classical pathway of C was activated by GEC-bound IgG Ab, inhibition of either Crry or CD59 enhanced cytotoxicity, whereas inhibition of both Crry and CD59 together was additive. Therefore, Crry and CD59 are present and functionally active in GEC. Crry restricts C activation via both alternative and classical pathways. When the classical pathway of C is activated, or when Crry function is inhibited, CD59 limits C5b-9-mediated cytotoxicity. Anti-Fx1A binds to both Crry and CD59, which may account for its ability to activate the alternative pathway in vitro, and for its superior nephritogenicity in vivo.