Acyl-CoA: monoglyceride acyltransferase (MGAT; EC 2.3.1.22) has been studied in human small intestinal mucosa by means of a spectrophotometric method based on the detection of liberated CoA employing 5,5'-dithiobis-(2-nitrobenzoic acid). With optimal assay conditions available the pH optimum was spread between 7.0 and 7.7 with a maximum at a pH of 7.4. Dependent on its concentration one of the substrates, palmitoyl-CoA, caused severe inhibition which was largely prevented by the addition of albumin. Using palmitoyl-CoA and 1-monooleoylglycerol as substrates specific activities were 23.0 +/- 8.4 in total homogenates compared with 92.9 +/- 28.3 nmol CoA released/min/mg protein in microsomal fractions from jejunal mucosa. Concerning the substrate specificity, a broad acyl-donor pattern exists with maximal activities for C10:0 to C16:0, the highest for C16:0. A preferential esterification of acyl-acceptors was shown for 2-monoacylglycerols as compared with the 1-isomers, and for monoacylglycerols with unsaturated versus saturated fatty acids. MGAT was mainly localized in the microsomal fraction. The properties of MGAT from human small intestine are discussed with respect to the intestinal enzyme from other species.