The activity of human milk on cell growth has been evaluated on two cell lines, MDCK and Caco-2. The proportion of human milk samples that reduced by half the growth of MDCK cells was of 36%. This inhibitory activity was associated with casein and not the whey fraction. Great variability was found in the degree of inhibitory activity depending on the milk sample. The susceptibility of Caco-2 cells to milk inhibitory activity was lower than that of MDCK. Bovine milk did not have any effect on cell growth, either as skimmed milk or as whey or casein. Morphology of cells incubated with active human casein showed abnormal features, such as chromatin condensation, reduced cellular volume and apoptotic bodies, and also fragmented DNA, which are all features of apoptosis.