Certain penicillin beta-lactamase antisera have been reported to enhance, rather than neutralize, the bacterial enzyme activity. A similar study with antisera to cephalosporin beta-lactamase from two different strains of Enterobacter cloacae was undertaken. The effects of the antisera from rabbits immunized with each enzyme preparation were evaluated by neutralization tests using a spectrophotometric method. The antisera were also examined by immunoelectrophoresis. From the neutralization tests, it was observed that either an inhibition or an enhancement of the cephalosporin beta-lactamase activity occurred in the presence of antisera produced in different rabbits under the same conditions and to the same antigen. The inhibiting antisera to the beta-lactamase from each of the two strains were shown to cross-react by neutralization and immunoprecipitin tests; similarly, the two enhancing antisera were also shown to cross-react. Either inhibition or enhancement of the enzyme activity could be obtained with the cross-reactions and with one strain these effects were even greater than with the homologous reactions. The immunoprecipitin results revealed apparent differences between the inhibiting and enhancing antisera.