The presence of circulating IgE-like antibody was found not to enhance the induction of joint lesions, of moderate or greater intensity, by intravenous injection of bovine serum, but did make mild joint lesions more frequent. There was a positive correlation between increased white cell effusion into the joint fluid and joint lesions of moderate or greater intensity. Different breeds of rabbit were shown to produce different incidences of lesions suggesting a genetic influence on the development of rheumatoid-like joint lesions. The Old English breed was found to be particularly sensitive.