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Envelope membranes were isolated from potato tuber amyloplast by a discontinuous sucrose density gradient and high speed centrifugation. These membranes catalyzed the transfer of [14C]glucose from UDP-[14C]glucose to endogenous sterol acceptors and, in turn, catalyzed the esterification of steryl glucosides with fatty acids from an endogenous acyl donor. The synthesis of steryl glucosides was stimulated in the presence of Triton X-100, while formation of acyl steryl glucosides was inhibited by the detergent. However, in the presence of an added sterol acceptor and Triton X-100, the inhibition of acyl steryl glucoside synthesis was overcome by the addition of phosphatidylethanolamine. The enzyme involved in steryl glucoside formation was solubilized by treatment of the envelope membranes with 0-3% Triton X-100. The solubilized enzyme had an almost absolute requirement for sterol acceptors.