Goat whey was hydrolysed by pepsin in an ultrafiltration membrane enzymic reactor coupled with a 30 kDa mineral membrane. Peptides collected in the permeate were resolved using reversed-phase HPLC. Their sequences were determined by amino acid analysis, second order derivative spectra analysis and mass spectrometry. Owing to the resistance of β-lactoglobulin (β-lg) towards pepsin, the majority of peptides identified were derived from α-lactalbumin (α-la). Pepsin showed a broad specificity of hydrolysis sites and generated a wide range of products from dipeptides to very large peptides containing disulphide bridges. The molecular masses of peptides resulting from α-la degradation were between 150 and 6900 Da: 36% were < 600 Da, 24% were 600–2000 Da and 40% were > 2000 Da.