Using flow cytometric analysis we investigated the distribution of major lymphocyte surface antigens in newborn infants. A total of 221 newborns entered the study, of whom 53 fulfilled our criteria of healthy mature neonates. Percentages of immunofluorescent-positive cells were as follows (median and range from 25th to 75th percentiles given): for CD1 3.8%; 2.3%-5.8%. CD2 60.9%; 52.4%-66.8%. CD3 57.5%; 50.5%-63.3%. TcRass 57.7%; 48.1%-60.0%. CD4 36.3%; 28.0%-42.6%. CD8 23.0%; 20.0%-27.4%. CD11a 56.3%; 46.3%-68.5%. CD19 12.1%; 8.6%-14.8%. CD20 10.9%; 8.4%-12.9%. CD25 2.6%; 2.1%-4.5%. CDw52 61.0%; 51.2%-76.1%. CD71 5.2%; 3.1%-9.3%. While the ranges for the percentage of immunofluorescent-positive cells were rather small, there was a wide variation in the absolute numbers of marker immunofluorescent-positive cells.