The rates and extents of hydrolysis of alphas- and beta -caseins from bovine, caprine, and ovine sodium caseinates produced by an enzymatic extract of the fruit of Opuntia ficusindica, (L.) Miller were evaluated and compared with those produced by a commercial animal rennet. A mechanistic model based on a pseudo-first-order enzymatic reaction, in the presence of first-order deactivation of the enzyme, was postulated and successfully fitted to the experimental data. The animal rennet exhibited higher enzymatic efficiency than the fruit extract, irrespective of the source (i.e., bovine, caprine, or ovine) and the type (i.e., alphas- or beta -casein) of substrate. The enzymatic efficiency (k(cat)/K-m) for alphas-casein ranged from 72 to 220 and from 43 to 65 L g(-1) h(-1), and for beta -casein from 242 to 742 and from 55 to 164 L g(-1) h(-1), for the animal rennet and the enzymatic extract of O. ficus-indica, respectively. Finally, it was observed that beta -casein from caprine and ovine caseinates was degraded by O. ficus-indica faster than its alpha (S) counterpart, but the reverse was observed for bovine caseinate.