The leukocyte left shift is commonly used as an adjunct to the early diagnosis of bacterial infection. Many different methods have been employed for its quantification, such as the absolute band count, band/seg ratio, band/total neutrophil ratio, and immature/total neutrophil ratio. We examined blood and bone marrow samples in groups of noninfected and infected neonatal dogs and human beings in order to determine which method most clearly reflects an increased call upon marrow neutrophil reserves and which correlates best with the presence and severity of infection. We found that the neutrophil ratios were more frequently abnormal during neonatal sepsis than was the the absolute band count. All subjects, canine and human, in whom the immature/total neutrophil ratio exceeded 0.800 were found to have depletion of the marrow neutrophil reserves, and those with the most profound depletion died. This study supports the concept that an elevated immature/total neutrophil ratio can aid in the diagnosis of bacterial infection in the newborn infant and suggests that the degree of elevation may serve as a method for detecting subjects at high risk for depletion of the marrow neutrophil reserves and death from sepsis.