Egg PC-[14C] cholesterol proteoliposomes containing 50 mol% SM (with respect to PC) were first treated for 30 min with either SMase C (100 mU) or SMase D (100 mU), or a sequential combination of the two, in the presence of 50 μM Mn2+ and 200 μM Mg2+, as described in Methods. The SMase reaction was stopped by the addition of 4 mM EDTA, and the substrate was then incubated with human LCAT (phenyl Sepharose eluate) for an additional 30 min. The LCAT reaction was stopped by the addition of ethanol containing unlabeled FC and CE, and the percent of labeled FC esterified was determined as described in Methods. All activities are expressed as percent of activity obtained with SM-free substrate. The values shown are mean ± SEM of 4–6 experiments.