A novel antibody against human carbonic anhydrase XII (CA XII) was developed and characterized. Its specificity for the human CA XII isoform was assessed and its ability to inhibit CA XII was found about 40 times higher than acetazolamide, a well-known CA sulfonamide inhibitor. The antibody was shown to decrease cell proliferation in cultured A549 lung carcinoma cells, making it an interesting molecule for targeting and treating some forms of CA XII overexpressing tumors. The possibility of conjugating this antibody with fluorescent markers for diagnostic purposes or with toxins or anticancer drugs for treating purposes makes it an interesting molecule for developing new combined approaches for cancer treatment.