Fermented milks (yogurt, kefir, sour milk), compared with milk for their non-protein nitrogen and free amino-nitrogen contents gave higher values prior to as well as 1, 2 and 3 h after pepsin digestion in vitro. In feeding trials of 6 weeks duration with diets based on casein, milk and fermented milks, respectively, those groups of weanling (50 to 60 g) male Wistar rats showed the maximum increments of body mass which had been fed the diets based on fermented milks. The highest values of the protein utilization index (increase in body mass in g per protein intake in g) were found in the animals kept on the fermented milk products. The favourable protein utilization and body mass increment on fermented milk diets are attributed to a better digestibility of proteins in these products.