Increased reactivity of rat alpha-foetoprotein with corresponding antiserum after 125I labelling.

Two rat alpha-foetoproteins (AFP) of different origin and purified according to two different techniques were labelled with 125I using an identical procedure. After labelling the reactivity of one of these AFPs was not modified whereas that of the other was markedly increased; this in spite of a greater purity made the latter unsuitable for the radioimmunoassay of rat AFP. The observed difference may be due to the technique of purification and brings additional evidence for the necessity of checking the reactivity of tracers.