Two variants of a radioimmunoassay for the bipyridylium herbicide Paraquat are described. Both employ antiserum raised to Paraquat-BSA which has been covalently linked to particulate solid-phase support media. The rapid assay for clinical use employs a [3H] Paraquat tracer, requires no agitation and yields results in the range 10--2500 ng/ml serum in 20 min from receipt of sample. The more sensitive assay, designed for research purposes, employs a 125iodinated tracer, requires 2 h continuous agitation but can detect Paraquat at 0.1 ng/ml in simple aqueous solution or 0.25 ng/ml serum. Results from rapid clinical assay agree well with the existing colorimetric method.