Depletion of the mature neutrophil reserve during bacterial sepsis is rare in adults but common in neonates; when it occurs, a fatal outcome is likely. Neutrophil reserve depletion was investigated in groups of premature, 1-day-old and 1-, 2-, and 4-week-old rats by measuring: (1) the size of the neutrophil storage pool, and (2) the proportion of this pool which was released from the storage compartment when a weight-standardized release stimulus was applied. It was found that the premature rat has a small neutrophil storage pool containing 1.29 +/- 0.07 X 10(6) cells/g body weight (mean +/- SE). This pool size increases to contain 4.35 +/- 0.23 X 10(6) cells/g in the 4-week-olds (p less than 0.001). With a standard neutrophil storage pool release stimulus, the premature rats depleted 68 +/- 4% of their neutrophil stores, vs. a depletion o only 13 +/- 6% of the stores in the 4-week-olds (p less than 0.001). The small neutrophil reserve and the exaggerated release of stored neutrophils in neonatal animals are factors which predispose neonates to neutrophil reserve exhaustion during bacterial sepsis.