A double antibody, semi-automated radioimmunoassay for serum prostatic acid phosphatase (PAP) is described, which uses the 125I-labelled N-succinimidyl-3-(4-hydroxyphenyl) propionate ester of PAP. This type of label has substantially higher immunoreactivity than that prepared using the chloramine-T method of radioiodination. Chromatographic purification of either label on Ultrogel ACA44 further improved immunoreactivity. The lowest detection limit (0·35 μg/l) was achieved with the chromatographically purified ester label.
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