The proteolytic activity of enzymes of psychrotrophic bacteria on casein and raw and ultra-high-temperature (UHT)-treated milk was studied by capillary electrophoresis (CE) in order to characterize the degradation products and evaluate the possibility of distinguishing them from those due to the action of chymosin on κ-casein. Casein hydrolysates with Pseudomonas fluorescens B52 proteinase, milk inoculated with P. fluorescens, and commercial UHT milks were studied. The degradation products included caseinomacropeptide (CMP), pseudo-caseinomacropeptide (pseudo-CMP; CMP lacking the 106 Met residue) and an unidentified third peak. These peaks were already present in some freshly prepared UHT milk samples, and a progressive increase in peak area was observed upon storage at 22 °C. This might give rise to false positive results when the presence of rennet whey is investigated by CE. However, low area ratios of pseudo-CMP to CMP might allow the presence of rennet whey solids to be suspected. This work received financial support from Project COR0035-94-1 (Comunidad de Madrid). I.R. acknowledges the Spanish MEC for a scholarship.