Volume and deformability of blood cells are important determinants of the microcirculation. Leukocytes are larger and considerably less deformable than erythrocytes. In our study, volume and deformability of polymorphonuclear neutrophils (PMN), lymphocytes, and monocytes in adults and full-term neonates were studied by means of a micropipette system. Neonatal immature granulocytes were also investigated. Membrane cytoplasm tongues were aspirated into 2.5-microns (diameter) micropipettes over a period of 1 min. Adult and neonatal PMN were totally aspirated into 5-microns micropipettes. Tongue growth and final tongue length of PMN were about twice those of monocytes and lymphocytes. At a pressure of -2 cm H2O, tongue growth of lymphocytes and monocytes was similar. At a pressure of -4 cm H2O, however, tongue growth of monocytes was faster and the final tongue was longer than those of lymphocytes (p less than 0.05). Cellular volume and deformation behavior of the different leukocyte subpopulations (PMN, monocytes, and lymphocytes) were similar in neonates and adults. Compared to mature neonatal PMN, immature neonatal neutrophilic granulocytes were significantly less deformable (final tongue length of 5.4 +/- 1.52 versus 9.3 +/- 1.48 microns at -2 cm H2O) and larger (421 +/- 68 versus 360 +/- 38 fL). The entry time of PMN into 5-microns micropipettes was similar in neonates and adults at aspiration pressures of -2, -3, and -4 cm H2O. We conclude that the deformability of neonatal and adult leukocytes is not different despite functional differences and that immature granulocytes may contribute to impaired microcirculation in neonates with severe septicemia or hypoxemia.