Avian oncornavirus group-specific antigen: detection and quantification by radioimmunoassay.

A double-antibody competitive-inhibition radioimmunoassay for avian group-specific antigen is described. A viral protein preparation, consisting primarily of gs-1, was labeled with radioiodine. Hamster and rabbit antisera were reacted with the labeled antigen, and the resultant antibody-antigen complexes were precipitated with the appropriate antiglobulin. Standard curves based on the inhibition of binding of labeled antigen to antibody after preincubation with unlabeled antigen were prepared. These showed the lower limit of sensitivity to be 2 to 10 ng of unlabeled protein when labeled antigen of 2,000 to 5,000 counts per min per ng of specific activity was used in the assay. Extracts of avian oncornavirus-infected cells, likewise, were able to inhibit binding of labeled antigen. This technique will be very useful for the quantification of small amounts of oncornavirus group-specific protein.