Seric levels of C reactive protein (C.R.P.) were determined in a series of 125 children under one month old. Of them, 75 were clinically healthy and 50 showed signs that suggested infection. In all this infants, in addition to C.R.P., different components of white blood cell differential count were determined, and a bacteriological study was undergone. Authors found high concentration of C.R.P. in 100% of children in which diagnosis of sepsis was bacteriologically confirmed. On the other hand, concentrations of this biological parameter were not substantially modified in newborns used in the control, nor in those other ones in which sepsis was not confirmed. Sensibility of C.R.P. showed itself significantly higher than other hematological indexes in study.