The distribution of normal cell counts (the reference range) has been determined previously for circulating neutrophils in infants from birth to 28 days of age. We have determined the reference ranges for the absolute peripheral blood lymphocyte, monocyte, and eosinophil counts obtained from 393 infants in this same cohort. Furthermore, white blood cell counts were obtained from three groups of infants with perinatal complications previously shown to be associated with abnormal neutrophil values (ABO incompatibility, n = 82; maternal hypertension, n = 68; neonatal sepsis, n = 140) and compared with the derived reference ranges; significant alterations in the distribution of cell counts were found in each of these groups at different times. Our data provide reference ranges for lymphocyte, monocyte, and eosinophil counts in the neonatal period and evidence of the effect of specific perinatal events on these cell counts.