The hydrolysis of isolated αs1- and β-caseins by Penicillium roqueforti aspartyl proteinase produced comparable quantities of pH 4·6 soluble N. The amount of non-protein nitrogen obtained with β-casein was clearly lower than that obtained with αs1-casein, showing that few low molecular weight peptides were released when this casein was hydrolysed. Electrophoresis of αs1-casein hydrolysates produced by aspartyl proteinase showed 5 bands of mobility close to or higher than that of αs1-casein. β-Casein hydrolysates gave 4 bands, 2 of which (βPrapl and βPrap2) showed low electrophoretic mobility. The products corresponding to βPrapl and βPrap2 were purified from a β-casein hydrolysate and identified as fragments Val98-Val209 and Glu100-Val209 of β-casein respectively. The occurrence of the βPrapl and βPrap2 bands in electrophoretic patterns obtained from sterile curd with aspartyl proteinase and controlled-flora curd, where P. roqueforti was the only micro-organism developing, showed the presence of aspartyl proteinase synthesis and activity in cheese. A band of very low electrophoretic mobility (βPrmpl) was present in electrophoregrams of controlled-flora curd inoculated with P. roqueforti. This band, resulting from the action of the metalloproteinase on β-casein, revealed that this enzyme was both synthesized in and active in cheese.