ExtractThe numbers of circulating T and B lymphocytes in seven children with whooping cough due toBordetella pertussisand eight control subjects were determined. All the children with whooping cough had an absolute lymphocytosis (mean 29,142/mm3vs. 5,225/mm3) and by surface marker criteria both T cells and B cells were increased (mean T cells, whooping cough 15,794/mm3vs. 3,516/mm3controls; mean B cells, 13,393/mm3whooping cough vs. 1,706/mm3controls). However, the ratios of T cells to B cells in the whooping cough (1.4) and control group (1.9) did not differ significantly. This proportional increase in both T and B lymphocytes indicates that whatever mechanisms are responsible for lymphocytosis inB. pertussisinfection affect both populations in a similar manner.SpeculationLymphocytosis in children with whooping cough results, primarily, fromB. pertussis-induced blockage of T and B cell re-entry into lymph nodes from the blood. The profound effect ofB. pertussison lymphocyte recirculation patterns might be of value in future attempts at “immunologic engineering.”