Immunophenotyping of blood lymphocyte subpopulations in neonates and young infants is hampered by the limited amount of blood that can be collected. Contamination of the flow cytometric "lympho-gate" by normoblasts and analysed erythrocytes, and therefore the underestimation of the relative frequencies of lymphocyte subpopulations, interferes with the precise calculation of absolute counts. A microassay was developed by adapting the lysed whole blood technique. Triple immunostaining in a single antibody staining step was used to reduce washing steps and cell loss. Introduction of a triple staining for CD71 (expressed by erythroid precursors), glycophorin A (GpA, expressed by all erythroid cells), and CD45 (expressed by all leucocytes) permitted the relative frequencies of normoblasts (CD71(+)/GpA+/CD45(-) population) and unlysed erythrocytes (CD71(-)/GpA+/CD45(-) population)to be identified and measured within the "lympho-gate" of neonatal cord blood samples. Particularly high frequencies were found (median: 31%) in cord blood samples from preterm neonates. These erythroid cells disappear rapidly by 1 week of age The relative frequencies of erythroid cells can be used to calculate correct lymphocyte subpopulation values. Using only 0.5-0.8 ml of blood, this micro- assay would also be suitable for rapid prenatal immunodiagnosis of congenital immunodeficiencies.