In a prospective randomized trial the clinical value of Bestatin, a low molecular weight immunomodulator, is being examined in patients having completed a full course of local radiation therapy for bladder cancer. At termination of irradiation (64 Gy in eight weeks) the patients are divided into two treatment arms: (i) 10 mg of Bestatin orally three times daily without breaks for at least one year and (ii) no further adjuvant treatment. Routine haematological monitoring of 68 patients for a period of up to two years did not reveal any differences between the two groups. Studies on the blood lymphocyte population, however, showed a significantly elevated frequency of cells forming rosettes with sheep erythrocytes after one month of Bestatin treatment. Upon continuation of treatment, however, this value declined and reached the level of the control patients at three months. The frequencies of lymphocytes with Fc-receptors for IgG or complement were unaffected by Bestatin. Spontaneous cytotoxicity against Chang cells appeared to be increased during the first three months of Bestatin treatment in those patients who survived for more than 18 months. No increase was observed in patients who died earlier. Although the above data seem to indicate that Bestatin should be administered at higher doses and intermittently instead of continuously, our preliminary results on disease-free survival in a limited patient material seem to be in favour of the Bestatin treated patients.