Alpha-1-antitrypsin Pi phenotyping was performed by thin-layer isoelectric focusing on samples from 1653 healthy white blood donors. The variants were confirmed by the acid-starch gel technique and crossed immunoelectrophoresis, with complete agreement between the two methods. The allele frequencies in this population were PiM, 0.9019; PiS, 0.0713;PiZ,0.0142; PiI, 0.0036; PiF, 0.0036; PiV, 0.0024. In addition, some rare phenotypes (MX, IS, LM) were noted. No difference was noted in the distribution of the variant alleles between males and females. The significance of the high frequency of the S allele is related to the ethnic origin of this population. The agreement of the results obtained by our technique and by acid-starch gel electrophoresis confirms the validity of Pi determination by thin-layer isoelectric focusing.