The present study constitutes the first finding of the calcium-binding protein S100B and of its mRNA in human milk, as revealed by a quantitative immunoluminometric assay, by Western blot analysis and by reverse transcription-polymerase chain reaction (RT-PCR) assay followed by restriction enzyme digestion. The concentration of S100B in milk is markedly higher than that observed in other biological fluids such as cord blood, peripheral blood, urine, cerebrospinal fluid and amniotic fluid. This finding could be related to a possible trophic role, which has been hypothesized for the protein.