A zone immunoelectrophoresis assay (ZIA) for human serum apolipoprotein A-I (Apo A-I) is described. In this immunologic technique, glass capillaries filled with agarose gel containing anti-Apo A-I are used. Apo A-I produces zones of immunoprecipitates with migration distances directly proportional to the apolipoprotein concentration. Two plasmas and their isolated high density lipoprotein (HDL) fractions, having different particle size distribution patterns have been analyzed for Apo A-I using rocket electroimmunoassay in parallel with ZIA. A good agreement between the two methods was obtained. With ZIA, Apo A-I complexed with lipids in HDL yielded concentration values that were unaffected by lipoprotein particle size and by delipidation. A standard curve, linear between 0.5 and 3.5 mg Apo A-I/1, was obtained with ZIA. A protocol for the preparation of an Apo A-I standard without prior purification of the apolipoprotein is presented. Using ZIA, 75 serum samples representing a heterogeneous study population were analyzed for Apo A-I. A mean value of 1.23 g/l with a relative standard deviation of 28% was obtained. The corresponding mean for total HDL cholesterol was 1.64 mmol/l (RSD = 61%).