The effect of zinc and cadmium on delta-aminolevulinate dehydratase of bovine erythrocytes stored at -30 degrees C for different times was determined. The results show a. storage of erythrocytes leads to an enhancement of the enzyme activity, which after six weeks is 165% (500 muM ZnCl2) respectively 220% (100 muM CdCl2) for red blood cells of calves, and after four weeks in 420% respectively 450% (same concentrations) for red blood cells of adult cattle, b. the older the samples are, the higher is the metal concentrations, needed for activation.