Water soluble derivatives of carboxypeptidase A (CPA: peptidyl-L-aminoacid hydro-lase, EC 18.104.22.168) were prepared by coupling the enzyme with periodate-oxidized polyvinyl alcohols (PVA) of varying acetyl content. Treatment of CPA-polyvinylalcohol conjugates prepared with PVA containing 12% acetyl groups with p-azobenzene arsonate led to red colored derivatives that exhibited at pH 8.2 a negative ellipticity band at 520 nm (θ520 = 0.9 x 104) and a positive ellipticity band at 430 nm (6430 = 1.75 x 104). These findings were qualitatively similar to the known effects of arsanylation on native CPA in crystalline suspension. In contrast, arsanylation of CPA-polyvinylalcohol conjugates prepared with PVA devoid of acetyl groups gave yellow colored derivatives that exhibited virtually no negative ellipticity band and a low ellipticity positive band (θ430 = 0.65 x 104). The specific esterase activities were not affected by any of the modification steps employed. In contrast, the specific peptidase activities of the various derivatives were lowered following each modification step. The data suggested that different confor-mational states could be imposed on azoTyr-248 at the active site of CPA by, presumably, hydrogen-bond type interactions involving this amino acid residue and different structural elements (acetylated or free hydroxyl groups) of the PVA side chains.