The performance characteristics of time-resolved fluoroimmunoassays (FIAs) for oestradiol and progesterone were evaluated in an in-vitro fertilization/embryo transfer (IVF/ET) programme and for use in differential diagnosis of early pregnancy. Comparisons were made with commercially available radioimmunoassays (RIAs) and the correlations between the results obtained were analysed. The possible clinical impact of the differences was assessed in 30 women undergoing ovarian stimulation for IVF/ET and also in 68 women presenting with suspected pathological gestation. There was good agreement between the performance characteristics observed in the present study and those claimed by the manufacturer. There was also a strong correlation between RIA and FIA within the working ranges of the assays. The differences in the results obtained with the two methods were within the normal limits of variation of the assays, and, consequently, the clinical relevance was minimal. In conclusion, for the clinical purposes in question, the FIAs perform at least as well as commercially available RIAs, and may, therefore, be regarded as an alternative to RIAs for many laboratories.