The effect of radiation sterilization on casein alone and in the presence of glucose or starch was assessed by means of nitrogen balance studies in growing rats and compared to the effect of heat sterilization. No decrease in protein digestibility and utilization was noticed in the irradiated samples nor did the presence of glucose or starch during processing cause and changes of these parameters. Following heat sterilization of casein in the presence of glucose there was a significant reduction in protein digestibility and Net Protein Utilization (NPU). These changes were accompanied by a drastic decline of available lysine. The inclusion of 3% agar-agar (aqueous solution) in the diet induced a drastic rise in endogenous faecal nitrogen losses and a corresponding decrease in apparent protein digestibility and NPU.