OBJECTIVE: To develop a new tumor marker immunoradiometric assay and to evaluate its preliminary clinical application. METHODS: With monoclonal antibodies Ma552 and Ma695 from Sweden, a sandwich immunoradiometric assay was developed with Ma552 as catching antibody and Ma695 as tracer. The Ma552 was coated on polystyrene beads, and Ma695 was labeled with 125-I. The reaction was one step at room temperature. RESULTS: Bmax/B0 of the standard curve was 82. The sensitivity of this assay was 0.3 u/ml, CV within assay and CV between assays were 8% and 10%, respectively. Serum level of CA 15-3 in 50 normal women was 11.3 +/- 3.9 u/ml with a false positive rate of 0% based on the cut-off value of 30 U/ml. In 40 patients with benign breast diseases the serum level was 9.6 +/- 5.8 u/ml with a false positive rate of 0%. In 65 cases of breast cancers in different stages it was 88.4 +/- 159.6 u/ml before treatments with a total positive detection rate of 50.8%. Metastases, especially, bone metastases caused significant serum CA15-3 elevation, with 100% positive rate(n = 9). The positive rate of recurrences of beast cancers was 80% (n = 5). CONCLUSION: The newly established immunoradiometric assay of CA15-3 is highly useful in the diagnosis, differential diagnosis, and monitoring of metastases and recurrences of breast cancer, and is superior to CEA.