A radioimmunoassay test system has been designed for measurements of free a-subunit (AS) of glycoprotein hormones in human blood serum with a sensitivity of 0.15 ng/ml. This test system revealed the absence of a direct correlation between the levels of free AS and levels of glycoprotein hormones in the blood sera of women with various endocrine profile, this being indicative of the specificity of this test system, on the one hand, and, on the other, confirming the possibility of independent secretion by the pituitary of free AS into the blood. Basal blood serum level of free AS in normal subjects is low: 0,9 ng/ml in women aged 17 to 30. The level of free AS secretion in the blood of many patients with nonfunctioning tumors of the pituitary was found increased, this demonstrating the diagnostic significance of measuring free AS as a marker of such tumors.