The effect of apo very low density lipoprotein (apo VLDL) and apoprotein C-II on the phospholipase A1 activity associated with lipoprotein lipase (E.C.3.1.1.3) was studied using purified bovine milk lipoprotein lipase. The enzyme degraded 14C phosphatidylcholine (PC) to 14C 2-acyl lysophosphatidylcholine at a rate of 0.28 +/- 0.01 nmol/min/ml and triolein at a rate of 20.3 +/- 0.4 nmol/min/ml in mixed emulsions of PC and triolein. The phospholipase activity and triacylglycerol lipase activity were both increased by the addition of apo VLDL and apoprotein C-II. After maximal activation, the rate of PC degradation was 1.19 +/- 0.02 nmol/min/ml and triolein degradation 64.4 +/- 0.4 nmol/min/ml. Activation of phospholipase A1 activity and triacylglycerol lipase activity occurred in parallel.