The magnetic circular dichroic (MCD) spectra of cobalt(II) sugstituted metalloenzymes have been studied and compared to a series of four-, five-, and six-coordinate cobalt(II) model complexes previously examined (T. A. Kaden et al. (1974), Inorg. Chem. 13, 2582). The MCD spectra of cobalt substituted carboxypeptidase A, procarboxypeptidase ta, and thermolysin are consistent with earlier deductions of tetrahedral coordination from absorption spectra and also with X-ray structure analysis. Inhibitors fail to alter their MCD spectra significantly. The MCD spectra of cobalt alkaline phosphatase and carbonic anhydrase are more complex and their pH dependence and alteration by inhibitors are discussed in terms of known cobalt(II) models.