Cytokeratins are polypeptides which constitute a subclass of intermediate filaments in epithelial cells. The serum tumour marker M3/M21 is based on monoclonal antibodies against the epitopes M3 and M21 of cytokeratin 18. In the present study, we measured M3/M21 serum levels in 50 patients with FIGO stage IB-IIB cervical cancer and in 50 control subjects using a two-site radiometric immunoassay directed against soluble fragments of cytokeratin 18. Median serum levels of M3/M21 in patients with cervical cancer and in normal controls were 70.6 U/ml (range 0-397.7) and 6.5 U/ml (range 0-205.2), respectively (Mann-Whitney U-test, P = 0.0001). Median serum levels of M3/M21 prior to therapy and 4 weeks after therapy were 104.2 U/ml (range 24.6-397.7) and 39.3 U/ml (range 0-234.7), respectively (Mann-Whitney U-test, P = 0.004). We found a significant correlation between elevated M3/M21 serum levels and metastatic disease in pelvic lymph nodes (Mann-Whitney U-test, P = 0.002). 24 patients relapsed after complete remission. In these patients, elevated M3/M21 serum levels before the detection of relapse by computed tomography was observed in 13 cases. Considering these preliminary results, further studies with an increased number of patients are justified to clarify the prognostic value and the monitoring abilities of M3/M21 in cervical cancer patients.