A capillary isoelectric focusing (CIEF) method using bare fused-silica capillaries filled with polyethylene oxide (PEO) and carrier ampholyte solutions in the pH 3.5-5.0 range has been developed for the identification of alpha(1)-antitrypsin (alpha(1)AT) phenotypes in human serum. This novel procedure was routinely applied to the study of serum samples of five controls whose alpha(1)AT phenotype was previously identified and of twelve subjects whose alpha(1)AT phenotype was unknown. The results obtained allowed us to confirm or identify the alpha(1)AT phenotype in all sera tested. This procedure seems particularly suitable for identification of alpha(1)AT variants associated with diseases of clinical relevance.