Two techniques for the iodination of methotrexate are describes, involving covalent linkage to the drug of 125I-labelled N-succinimidyl-3-(4-hydroxyphenyl)propionate (Bolton and Hunter reagent), or to 125I-labelled histamine. A rapid highly specific radioimmunoassay for methotrexate was developed, employing a specific antiserum covalently linked to magnetisable particles, and 125I-labelled methotrexate as tracer. Incubation times for the assays were 60 and 10 min for the Bolton and Hunter reagent-linked methotrexate and 125I-labelled histamine-linked methotrexate respectively. Separation of bound from free antigen was achieved by a rapid magnetic separation system. Results obtained for serum samples correlated closely with those using an enzymatic (dihydrofolate reductase) competitive protein binding assay for methotrexate. A major advantage of the assay is its potential for processing large numbers of samples rapidly, making it highly suitable for routine clinical use.