The technical performance and clinical usefulness of the newly developed Elecsys CA 125 II assay (Boehringer Mannheim) was evaluated in a multicenter study. Imprecision studies were carried out using control sera and human pool sera with CA 125 concentrations from 11 to 1026 U/ml. Within-run CVs between 0.7 to 4.8% (median 1.7%) and between-day CVs between 2.4 to 10.9% (median 5.7%) were found. Method comparison studies with Enzymun-Test CA 125 II carried out in four laboratories yielded slopes between 0.94 to 1.07 and intercepts3 U/ml. A good comparability of the Elecsys CA 125 II assay was also found with one MEIA and the Centocor" IRMA. For a second MEIA and a second IRMA the slopes were 1.23 and 1.42, and the corresponding correlation coefficients were 0.987 and 0.977, respectively. The Elecys CA 125 II concentrations are clearly related to the tumor stage of ovarian carcinoma patients. The maximum of diagnostic efficiency of ovarian carcinoma patients compared with patients of benign gynecological diseases is reached at 150 U/ml with a specificity of 93% and a sensitivity of 69%. Follow-up studies of ovarian carcinoma patients reflect the status of the disease and the effect of various therapeutic applications. The technical and clinical evaluation of the Elecsys CA 125 II assay show a superior analytical performance with a broad measuring range up to 5000 U/ml and a short measuring time of 18 minutes.