The detection of low amounts of 3H-, 14C- or 35S-labeled compounds can be difficult with x-ray films. Here we report the benefits of blotting labeled molecules that have been electrophoretically separated to a membrane prior to autoradiographic or fluorographic detection. Blotting allows a reduction, by a factor of 5-10, in exposure times compared with dried gels. When combined with a rapid and safe alternative to the impregnation of blots with liquid scintillant, a further 3-fold to 5-fold increase in sensitivity is achieved. Bands containing as low as 1 Bq 14C or 100 to 10(3) Bq 3H could be detected after 24 hours of exposure. This enhancement in sensitivity may lead to new applications of 3H and 14C.