The authors' first experience in the determination of plasma somatomedin C concentration by the radioimmunologic method without extraction was put forward. The method itself was checked up through the routine indicators and the range of normal limits was established by the determination of this concentration in the plasma of 15 healthy persons. Values of plasma and serum determination greatly differentiated. By the determination of somatomedin C concentration and growth hormone during the insulin test in 8 persons it was shown that somatomedin C concentration did not change by the abrupt and short changes in growth hormone concentration. A good correlation was made by the comparison of sometomedin C and growth hormone basal concentration in 16 samples whereby the regulatory role of growth hormone in somatomedin C excretion was confirmed. In conclusion, radioimmunologic determination of sometomedin C in diagnostics and the therapy of growth disturbance was emphasized.