We describe a T3 uptake test that uses magnetic albumin microparticles to separate free from bound [125I] T3. The test uses 0.1 ml serum in a total assay volume of 1.1 ml, and is performed in barbital buffer of pH 7.6, ionic strength 0.2. The accuracy of the test is not affected by the following ranges: volume, 1-3 ml; incubation time, 15-120 min; and incubation temperature, 4-35 degrees C. We describe a magnetic separator device that allows simultaneous handling of 100 assay tubes with minimum manipulation. The intra-assay and interassay coefficients of variance were 2.87% and 5.20%, respectively.