Specific radioimmunoassay was developed for detection free glycoprotein hormone alpha-subunit (AS), by using AS prepared from highly purified human thyroid-stimulating hormone (TSH). The assay showed that the basal content of free serum AS in the majority of 24 females with hormonally pituitary tumors secreting somatotropic hormone (STH, growth hormone), prolactin or either and in 24 females with unfunctioning pituitary tumors was significantly higher the mean one in 12 females matched for the same age who had no endocrine diseases (control). The cultured cells of STH- and P-secreting tumors released excessive quantities of free AS along with STH and P into the medium with low glycoprotein holo-hormone levels. The content of AS changed little in 12 patients with non-functioning pituitary tumors despite the greatly increased serum TSH levels in response to the hypothalamic stimulator thyroid-releasing hormone. The findings suggest that free AS secreted into blood in excess despite hypothalamic control may be regarded as a biochemical marker of pituitary tumors.