The subunit and amino acid composition of the enzyme that catalyses triacylglycerol synthesis was determined for the first time from plant tissues. Diacylglycerol acyltransferase (acyl-CoA:1,2-diacylglycerol O-acyltransferase, EC 2.3.1.20) purified from germinating soybean (Glycine max L. Merr. cv. Dare) cotyledons, dissociated into three nonidentical subunits having apparent molecular masses of 40.8, 28.7, and 24.5 kDa. The respective subunits occurred in a 1:2:2 molar ratio in the native enzyme. Five peptides in that molar ratio were assumed to constitute a monomer having a putative molecular mass of 153.1 kDa. Based upon the apparent molecular mass of purified diacylglycerol acyltransferase after delipidation (1539 kDa), there was a high probability that the complete structure of the native enzyme from soybean contained ten identical monomers. The polarity index of each subunit was less than 21%, far below the 40% boundary reported for membrane bound proteins. Hydrophobic amino acids accounted for greater than 48% of the composition in each subunit. It was predicted from these data that the native enzyme contained 12,525 amino acid residues, and that the two smaller subunits were more deeply embedded in the membrane than the 40.8 kDa subunit. Attempts to reactivate the denatured or delipidated protein were not successful.