Based on knowledge of the changes produced in the erythrocyte sedimentation rate, segmented polymorphonuclears, non-segmented polymorphonuclears and platelets in patients with septicemia, a mathematic devise was developed and called septicemia index which correlated the four mentioned factors and made it possible to situate patients clinically suspicious of septicemia within three different groups: 1) positive, 2) probable and 3) negative. The method showed to be statistically significant in the early detection of septicemia in the infant and in cases without septicemia (p less than 0.01). The method may be carried out in general laboratories and no special techniques are required. The results may be obtained within a few hours in opposition to cultures that require from 3 to 5 days.