The conversion of biliverdin to the bile pigment bilirubin is catalyzed by biliverdin reductase, a cytosolic enzyme with two pH optima with different cofactors. The enzyme is assayed at pH 8.7 with NADPH as a cofactor and at 6.75 with NADH. The production of bilirubin is detected as described in this unit with a spectrophotometric assay. The enzyme can be qualitatively assayed by immunoblotting and changes in reductase isoform expression can be detected by two-dimensional electrophoresis.