We evaluated the Stratus (American Dade, Miami, FL), an automated immunofluorometric assay system, for the quantification of human choriogonadotropin (hCG) in serum or plasma. The assay is based on the "sandwich" (two-site) immunoassay methodology: use of two monoclonal antibodies, one specific for the alpha subunit and the other for the beta subunit, results in an assay that is specific for the intact hCG molecule. Results for the first sample are obtained in 7 min; subsequent additional values are produced at 1-min intervals. Inter-run precision (CV), estimated from replicate determinations of sera, was 4.5% at an hCG concentration of 38 int. units/L, 4.9% at 114, and 6.1% at 194. Intrarun CV was less than 2% at all three concentrations. Correlations of results for 127 specimens analyzed in duplicate with the Stratus (y) and by a radioimmunoassay (x) for beta hCG (Gamma Dab M [cf931125I] beta-hCG, Travenol-Genentech Diagnostics, Cambridge, MA) yielded the following regression equation: y = 0.969x - 6.0 (r = 0.995). The Stratus immunofluorometric system provides a rapid and convenient assay of hCG in serum or plasma.