SUMMARYMethods for the accurate assay of rabbit papilloma virus and its antibody are described. Viral infectivity is titrated by a technique in which up to fifty discrete tumours may be counted on a single inoculation site. Neutralizing antibody is measured with considerable accuracy by determining the dilution of serum producing a 50 p.c. reduction in tumour count. A complement fixation test designed to economize on reagents permits the quick and accurate titration of RPV antigen and antibody.Some of the relationships between tumours, infectious virus, antigen and antibody in naturally and experimentally infected rabbits are examined.