Immunosuppressive acidic protein (IAP) was determined in sera of patients with gynecologic tumors using the single radial immunodiffusion method. The normal limit of IAP of 490 micrograms/ml was derived from the mean value + 2 SD of IAP in 150 healthy females. Among 141 patients with gynecologic cancers, serum IAP was elevated in 87 patients (62%). Among 190 patients with benign tumors (98 uterine myoma, 92 benign ovarian tumors) serum IAP was elevated in 14 patients (7%). Elevated levels of IAP were recognized in 43% of 77 patients with cervical cancer, in 55% of 11 endometrial cancer patients, and in 91% of 53 ovarian cancer patients. The frequency of elevated levels showed a tendency to increase with advancing stage of disease. In ovarian cancer elevation of IAP was observed even in early stages. All of 13 patients with recurrent cervical cancer had elevated IAP while only 6 of 35 (17%) previously treated patients without evidence of recurrence had elevated IAP. Immunosuppressive acidic protein determinations may be useful in monitoring the recurrence of cervical cancer. The measurement of serum IAP as a marker for gynecologic cancer is recommended as an addition to diagnostic procedures.