Procalcitonin (PCT) is one of the precursors in the synthesis of calcitonin in thyroidal C-cells and other neuroendocrine cells. PCT and other calcitonin precursors in serum are present at less than 50 pg/ml in healthy individuals, but are highly elevated in serum where conditions leading to systemic inflammatory response syndrome or sepsis prevail. We measured PCT concentrations in milk and serum samples taken from 9 healthy women after delivery. PCT concentrations were below 10 pg/ml in serum samples, but were more than 100 times as high in the corresponding milk samples. PCT in milk reached a maximum within the early days after delivery, with a median peak concentration of 2310 pg/ml (range 223 - 4224 pg/ml) at day one and 2442 pg/ml (range 952 - 4488 pg/ml) at day two, then declining over the next days to a median concentration of 747 pg/ml (range 443 - 1656 pg/ml) at day 10 (p = 0.012, by Friedman ANOVA). PCT values reached a steady state of 504 pg/ml median value. Mature calcitonin values measured in parallel with a specific assay were not above the normal range of 10 pg/ml in any samples measured. The strong discrepancy between serum and milk PCT suggests that PCT (but not mature calcitonin) is synthesised in the breast of healthy mothers after delivery. The precise mechanism and the physiological relevance are unclear. Since PCT levels increase drastically in serum from patients suffering from sepsis and related conditions, and since PCT has been ascribed a pro-inflammatory function, we propose that milk PCT might contribute to the activation of the developing neonatal immune system. Similar speculations were proposed for a variety of other pro-inflammatory cytokines, which had comparable kinetics in human milk.