Two methods of assaying alpha interferon (JFN-a) were compared during an experiment aimed at determining whether IFN-a crosses the human placenta. Human placentas, collected after delivery following a normal pregnancy to term, were catheterized on both sides: fetal and maternal. The IFN-a was introduced in known amounts in the maternal circulation and was assayed in the efferent fetal fluid. The following two detection methods were used: radioimmunoassay by competition with [1251]IFN-ce and assay with a biological system in which IFN-ce protected Madin-Darby bovine kidney cells from destruction by vesicular stomatitis virus. The results obtained by the two methods were in perfect agreement for the efferent fetal fluid samples. They showed the absence of placental transfer of IFN-a. The biological method was found to be more sensitive than radioimmunoassay for low IFN-a titers (< 10 IU/ml) but was less reproducible, probably owing to the use of twofold dilutions. The specificities of the two methods were similar and their practicalities were equivalent; the biological method, however, was less costly. The study illustrates the complementarity of the two methods, which were based on different principles. The agreement obtained between the two methods provides a clear confirmation of the experimental results.We report a comparison of two methods for the assay of alpha interferon (IFN-a). Assay of any substance with a specific biological activity can generally be done by several types of techniques. If an antibody (Ab) against the substance is available, it can be used in quantitative immunoassays such as radioimmunologic, immunoenzymologic, and immunofluorometric assays. The activity of the substance can also be measured if investigators have available a suitable biological system which will show a quantifiable modification upon the addition of the substance. The first type of technique is based on recognition of an epitope of the molecule. The second type, whose implementation is often more problematic, is based on the biological activity that develops. In the latter case, it is often difficult to eliminate all interactions between susceptible substances to bring about the same effects (false-positive results) or to inhibit the effect (false-negative results). We opted for a double IFN-a assay: radioimmunology (12) and a biological method described by Lebon et al. (11) that uses the protective effect of IFN-a on Madin-Darby bovine kidney (MDBK) cells, which are sensitive to the vesicular stomatitis virus (VSV) (1, 6).Transmission of the AIDS virus (human immunodeficiency virus type 1) from mother to fetus is currently a problem of some concern since in Europe the risk is estimated to be 14% (4, 15), and in Africa the risk is estimated to be 40% (8). In order to decrease this type of transmission, treatment with the combination of zidovudine and IFN-ct has been considered in pregnant, human immunodeficiency virus type 1-seropositive women (7,9). Adoption of such a protocol depends on prior knowledge of the possible...