The alternative pathway C3 convertase (C3b,Bb) is a Mg-dependent, labile enzyme with a t/2 of 3 min at 37°C and of 14 min at 24°C (at half physiological ionic strength). To stabilize the enzyme. metal ions of the lanthanide series were tested. Formation and decay of the enzyme as well as binding of radiolabeled Factor B, Factor H or properdin to C3b were measured using C3b-bearing sheep erythrocytes (EC3b). Binding of Factor B to EC3b in presence of 40 μM gadolinium (Gd) was two to three times greater than in presence of 1 mM Mg. Binding of Factor H and of properdin to EC3b was partially inhibited by Gd. Although it enhanced Factor B uptake by EC3b, Gd was unable to substitute for Mg in enzyme formation by Factor D and completely inhibited (at 10 μM) Mg-dependent enzyme activation. However, the preformed enzyme was not inhibited by Gd. Instead, exposure of EC3b,Bb to 40-100 μM Gd increased the t/2 at 37°C from 3 min to 12-28 min, and at 24°C from 14 min to 32min. The slow decay of the enzyme correlated with slow release of Bb. Similar enzyme stabilization was observed using terbium, ytterbium, dysprosium and lanthanum. The Gd stabilized enzyme was also less susceptible to control by Factor H and properdin than the unstabilized enzyme. Furthermore, Gd protected surface bound-C3b from being cleaved by Factor I. The Gd effects were instantaneously reversed upon addition of 10 mM EDTA. Thus, Gd is able to stabilize preformed C3b,Bb and to render the enzyme refractory to control by Factors H and I.