Abstract Micellar casein (MCN) and β-casein (β-CN) are precursors for peptides with angiotensin-I-converting enzyme- (ACE-) inhibitory effect, but to be effective these have to be available as individual peptides and this may be accomplished by a subsequent separation step. In this work, hydrolysates of MCN and β-CN were used as the starting material for ultrafiltration experiments. Molecular mass profiles were obtained and peptides were identified and characterised. The effect of raw material on peptide generation and separation was studied in terms of mass flow and mass balance. It was established that ultrafiltration allows the separation and concentration of ACE-inhibitory peptides with transport rates up to 15.3 ± 1.2 g h−1 m−2 and 17.6 ± 0.6 g h−1 m−2 for MCN and β-CN, respectively. The peptide fractions isolated by ultrafiltration showed a significant increase in ACE-inhibitory activity up to IC50 109 μg mL−1 and 98 μg mL−1.