Various leukocyte parameters, i.e., leukocyte alkaline phosphatase (LAP) activity, absolute segmented polymorphonuclear count (segs), absolute band count and bands/segs ratio were studied in 15 full-term newborns with bacterial infections, 15 full-term newborns with bacterial infections, 15 full-term newborns with viral illnesses, and 10 age- and weight-matched control newborns. Mean LAP scores were 87.5 in the control group, 160.3 in newborns with viral illnesses, and 232.7 in newborns with bacterial infections. The differences between each group were statistically significant. The mean band count and bands/segs ratio were 65.6/mm3 and 0.013 in the control group; 842/mm3 and 0.18 in newborns with viral illnesses, and 2,207/mm3 and 0.37 in newborns with bacterial infections. LAP was more sensitive than other parameters in distinguishing the three groups. The data indicate that an LAP score of > 200, a band count of > 500/mm3 and a bands/segs ratio of > 0.3 are highly suggestive of bacterial infection.