Acyl-CoA:1-acylglycerophosphate acyltransferase activity was found in porcine erythrocyte membranes. However, the membrane preparations did not catalyze the acylation of either 2-acylglycerophosphate or 2-acylglycerophosphocholine. The 1-acylglycerophosphate acyltransferase and the known acyl-CoA:1-acylglycerophosphocholine acyltransferase systems differ in their specificities for acyl-CoAs and in their stabilities to detergents. Diacylglycerol lipase and monoacylglycerol lipase activities were also detected in porcine erythrocytes. These two activities appear to be catalyzed by different enzymes inasmuch as diacylglycerol lipase but not monoacylglycerol lipase was completely inhibited by divalent cations. The diacylglycerol lipase was relatively specific for the 1-position yielding 2-acylglycerol. The monoacylglycerol lipase hydrolyzed 1-acylglycerol and 2-acylglycerol at comparable rates. Phosphatidic acid was dephosphorylated to form 1,2-diacylglycerol but the acyl groups of phosphatidate were not hydrolyzed significantly by the erythrocyte membranes. Thus, the origin of 1-acylglycerophosphate, a substrate for the newly described enzyme, acyl-CoA:1-acylglycerophosphate acyltransferase, in mature erythrocyte could not be ascribed to action of diacylglycerol lipase, glycerophosphate acyltransferase, or phosphatidate-specific phospholipase A. 1-Acylglycerophosphate may be supplied extracellularly or the 1-acylglycerophosphate acyltransferase activity in erythrocytes may be a remnant of de novo phosphatidate synthesizing system of reticulocytes.