The individual rate constants for acylation and deacylation (k2 and k3, respectively) of the class C beta-lactamase of Enterobacter cloacae 908R by ampicillin and carbenicillin have been determined. For several other beta-lactams, the value of k2 was too high to be determined and the k2/k3 ratio could be larger than 10,000. Branched pathways were also shown to occur with several penicillins and cephalosporins.