A therapeutic trial of transfusions with polymorphonuclear leukocyte concentrates was performed in newborn infants with bacterial sepsis proven by blood culture. With each transfusion, 20 ml/kg of a preparation obtained by continuous flow filtration leukapheresis, and containing 0.5 to 1 x 10(9) WBC, with less than 6% lymphocytes, was administered. Twenty newborn infants with sepsis received from 2 to 15 PMN transfusions. Results were compared with findings in 18 newborn infants with sepsis admitted during the trial period, and not treated because of unavailability of the PMN preparation (Group B). Infants with fulminant illness were excluded from both groups. Groups A and B were similar with respect to clinical characteristics and to etiology (in the majority cases a highly antibiotic-resistant Klebsiella). The mortality rate was significantly lower in Group A than in Group B in the whole series (10% vs 72%, P < 0.001), and also in the subgroups with birth weight equal or below 1,500 gm (10% vs 91%, P < 0.001). Major complications and associated conditions (i.e., necrotizing enterocolitis, meningitis, pneumonia, peritonitis, osteoarthritis, disseminated intravascular coagulation) were observed in 12 patients of Group B, and in only three infants of Group A. Untoward effects attributable to PMN transfusions were never observed. PMN transfusion was a highly effective therapeutic tool in our population of infected newborn infants.