Rates of hydrolysis of 4-, 3-, and 2-nitrophenyl acetate and 4-nitrophenyl propionate catalyzed by wild-type and mutant forms of human carbonic anhydrase II have been measured. The results show that the mutations Tyr7-->Phe and Ala65-->Leu lead to activity enhancements with all the investigated substrates, but there is no significant effect on the specificity. In contrast, some mutations at sequence position 200 have large effects on specificity. For example, while the mutation Thr200-->Gly results in a threefold increase of the rate of hydrolysis of 4-nitrophenyl acetate, the activity is enhanced 10 times with the meta-substituted substrate and 380 times with the ortho-substituted substrate. These results are interpreted in terms of the removal in the mutant of a steric interference between the 2-NO2 group, in particular, and the side chain of Thr200. Mutants involving residues lining a hydrophobic pocket near the catalytically essential zinc ion have also been investigated. The most pronounced effect on specificity was found for the Val143-->Gly mutant. This mutation leads to a sixfold decrease of the rate of hydrolysis of 4-nitrophenyl acetate but a 20-fold increase of the activity with the propionyl ester as substrate. These results suggest that the side chain of Val143 interferes sterically with the acyl moiety of 4-nitrophenyl propionate. Based on these results, we have constructed a hypothetical model of the location of these ester substrates in the enzymic active site.