In order to investigate the possible involvement of the antigen CA50 in patients with colorectal carcinoma， the carbohydrated antigen CA50 in serum was examined in 30 normal individual， 27 patients with benign colorectal diseases and 66 patients with colorectal carcinoma by immunoradiometric assay (IRMA). The results showed that the serum CA50 in patients with colorectal carcinoma was significantly higher than that in patients with benign colorectal diseases and normal individual (Ｐ＜0.01). It was significantly declined after radical operation (Ｐ＜0.01). However， no significant change was noted after palliative operation (Ｐ＞0.05) and elevation was noted in patients with tumor recurrence. The results suggest that the measurement of serum CA50 may be an useful marker for diagnosis and prognostic evaluation in patients with colorectal carcinoma.