&NA;The monoclonal antibody (MAb) Ki‐67 detects a nuclear antigen expressed by proliferating cells during the entire cell cycle. In contrast to conventional histological techniques, the use of MAb Ki‐67 on frozen sections or cytological smear preparations allows direct determination of the growth rate of tumors routinely. Sixty‐two pituitary adenomas were investigated by use of the MAb Ki‐67 in a two‐step avidin‐biotin‐peroxidase complex technique. The proliferation activity ranged from 0.1 to 2.8%. There was no significant difference between the proliferation and hormonal state of the adenomas. Adenomas for which there was histological evidence of dural infiltration, however, showed a statistically significant higher proliferation activity (P< 0.05) compared to noninvasive adenomas. (Neurosurgery25:927‐930, 1989)