We examined an immunoassay for estradiol (E2) on the Immulite-an automated, random access chemiluminescent immunoassay system-to determine its accuracy and precision required for in vitro fertilization (IVF) studies. The assay, which has a reportable range from 73 to 7300 pmol/L, demonstrated good linearity under dilution, a detection limit of 44 pmol/L, and interassay CVs of 12.6% and 7.6% at 466 and 6164 pmol/L, respectively. In a retrospective analysis of 545 serum samples, the assay showed adequate agreement with an antibody-coated-tube RIA. The two E2 assays showed good agreement, even on samples from patients receiving a variety of different estrogen replacement therapies. Longitudinal studies of individual IVF cycles showed good parallelism between the automated system and the RIA, and results by the automated assay correlated well with the total number of follicles.