Blood cadmium and plasma zinc were measured in a series of 47 patients admitted for myocardial infarction (MI) and in 37 control subjects divided into 3 groups. Certainly increased blood cadmium values, for the MI patients, were found in 33%. The mean blood cadmium value of the MI series was significantly (P less than 0.001), but not invariably, higher against the control subjects. The difference between the zinc values of the series was studied in 35 MI patients with raised serum enzyme activity. In MI patients, the mean plasma zinc content was significantly (0.001 less than P less than 0.01) lower than in healthy controls. Of the enzymes, the highest positive correlation was found between the ratio of blood cadmium to plasma zinc and the GOT activity. Of the ratio, the MI patients had values in excess of the controls range in 43%, and values within the limits of the healthy controls in 40%. Therefore, the rise in the ratio is of limited value for the diagnosis of myocardial infarction in an individual patient.