Abstract
A radioimmunoassay for the antigens of the mouse mammary tumour virus has been developed. ^125^Iodine‐labelling of intact virus [derived from mammary tumours of (C3H × 020) F1 mice] followed by ether extraction and separation of the ether and water layers resulted in a separation of the labelled material into two main groups of antigenic components. The intact labelled material from each group was separated from viral debris and other possible contaminants by affinity chromatography. The antigens of the ether phase were proved to belong mainly to the viral nucleoid whereas the water phase contained mainly envelope antigens. No type‐specific antigens could be demonstrated in either of the phases. Radioimmunological assessment of plasma revealed that in the plasma of MTV‐S‐ and MTV‐P‐positive animals viral antigens were only measurable when palpable mammary tumours were present, whereas in MTV‐L‐positive tumour‐bearing animals some negative samples were found. In milk of individual nontumour‐bearing mice a close correlation was found between the expression of viral antigens and the generally accepted presence of virus in the strain of mice. In milk, viral antigen levels tend to increase with parity with a possible decrease after a finite number of pregnancies. In a pilot study the presence of MTV antigens could also be demonstrated in epididymis extracts of male GRS/A mice. Genetical analysis of the low‐virulent MTV‐L by radioimmunoassay of milk is in progress.