The activity of uroporphyrinogen III cosynthetase may be determined in a direct assay system in which preuroporphyrinogen is generated from porphobilinogen by the action of porphobilinogen deaminase. The basis of the assay relies on the much faster enzymatic conversion of preuroporphyrinogen into uroporphyrinogen III by cosynthetase compared with the non-enzymic decomposition of preuroporphyrinogen into uroporphyrinogen I. The amount of porphyrinogen (measured as porphyrins) formed from porphobilinogen in the presence of the cosynthetase, after subtraction of the porphyrin formed with deaminase alone, gives a direct assessment of the uroporphyrinogen III and therefore of the cosynthetase activity. The assay may be utilized for the determination of pure cosynthetases and for cosynthetases in crude systems such as whole blood haemolysates or liver homogenates. The assay can be accomplished in a fraction of the time required for previous methods.