The concentrations of various clinical-chemical substances in the capillary and venous blood serum of apparently healthy adults (20-30 yr) were examined in the fasting state. Total protein, bilirubin, calcium, sodium and chloride concentrations were significantly lower (less than or equal to 5%) in capillary than in venous serum. In non-hemolytic capillary sera, the concentration of potassium was nearly the same as in venous samples. Inorganic phosphorus and urea concentrations were identical in both specimens. There was a tendency for glucose concentrations to be higher in capillary than in venous serum. It is concluded that capillary and venous blood serum can be used interchangeably only for certain purposes. It should be emphasized that these results are valid for apparently healthy adults in the fasting state; other population samples remain to be investigated.