As a part of our program to assess the effect of blood components to the drug delivery capabilities of phospholipid vesicles, we have studied the binding of human high density lipoprotein A-1 protein with phosphatidylcholine vesicle. Binding experiments were conducted by using the ultracentrifugation method with phospholipid to protein molar ratio between 1000 to 5000. It was found the extent of binding increased slowly in the range of phospholipid to protein ratio 2500 to 1000. In order to monitor the size change induced by binding, the complexs between apo A-1 and phospholipid were passed through a sepharose CL-4B column. There was no change in vesicle size in the range of phospholipid to protein ratio 3500 to 5000. On the other hand, an appreciable size decrease was observed below the ratio of 3000. There observation suggest that at high apo A-1 concentration, the vesicles are converted into a form of smaller size and that there two form of phospholipid aggregates have differed apo A-1 binding behavior. The binding parameters of apo A-1-phospholipid vesicle interaction were determined from the double reciprocal plot. This plot was also consistent with the above conclusion. Permiability test was performed to determined the effect of added apo A-1 and the temperature. There was a 56\% increase in the permeability by increasing the temperature from $37\,^\circ\!C$ to $44.5\,^\circ\!C$. In presence of $0.24\mu g/ml$ apo A-1, the increase in the permeability was 58\% over the control value in the absence of the apo A-1.