Tissues from 30 human pituitary adenomas are monolayer-cell-cultured in vitro. Hormone secretion of GH, PRL, TSH, LH and FSH by cells into medium is detected by radioimmunoassay. The pattern and amount of hormone(s) in the medium are used to determine the nature of the cells and thus to establish functional classification of pituitary adenomas. The results show that cell culture technique provides an easy and suitable mode for investigating the nature of pituitary adenomas. Hormone radioimmunoassay of culture medium is precise and reliable and represents the whole adenoma tissue. Further studies can lead to clearer understandings of the pathology of pituitary adenomas.