An assay for the quantitation of the serum levels of free and whole molecule-associated beta-subunit of human chorionic gonadotropin on the Abbott IMx analyzer is described. The assay detects human beta-chorionic gonadotropin with a sensitivity of approximately 0.5 IU/l while showing no cross-reactivity with follitropin (1000 IU/l) or thyrotropin (2.0 IU/l), and 0.02% cross-reactivity with lutropin (1000 IU/l). Haemoglobin (7.50 milligrams), bilirubin (0.50 milligrams), and triacylglycerols (10.6 milligrams) did not interfere with the assay. Pooled within-run, between-run, and total assay CVs were less than or equal to 5.2%, and less than or equal to 2.7%, and less than or equal to 6.3%, respectively. Values obtained with this assay correlated well (r = 0.98, n = 228) with those values obtained using the Hybritech TandemR-R HCG (Total Beta-HCG) IRMA. The normal range of the assay was found to be less than or equal to 5 IU/l (n = 311). The assay protocol provides results for up to 23 serum samples in approximately 47.5 minutes with the ability to report the human beta-chorionic gonadotropin concentrations of 5 specimens in approximately 15.5 minutes. We conclude that this is an acceptable assay for monitoring human beta-chorionic gonadotropin levels associated with normal pregnancy, reproductive pathology, and reproductive technology such as in vitro fertilization or embryo transfer.