Abstract.Gmelig‐Meyling, F., Dollekamp, I., Zegers, B. J. M. and Ballieux, R. E. (University Children's Hospital “Het Wilhelmina Kinderziekenhuis” and Department of Clinical Immunology, University Hospital, Utrecht, The Netherlands). Lymphocyte subpopulations in neonates, young children and adults as detected by six cell surface markers. Acta Paediatr Scand, 69: 13, 1980.—In this study, six surface marker tests were performed on lymphocytes from normal individuals of three age groups: neonates, children from 0 to 2 years of age, and adults. Determined were the proportions of T cells binding sheep red blood cells, of B cells carrying surface immunoglobulins or binding mouse red blood cells, and of lymphocytes bearing receptors for IgM, for IgG or for complement. The T cell percentage appeared to increase with the age; the percentage of B cells was highest in the children's group, as determined by both marker tests. Neonates had a much lower proportion of IgM‐receptor bearing cells than older individuals. The variation in the percentages of T cells and of IgM‐receptor bearing cells in the young age groups was relatively large as compared to adults. The significance of these observations is discussed; they may be of value for the proper evaluation of results obtained in diagnostical tests on neonates and yo