Random one-way microtiter mixed lymphocyte cultures between 43 rheumatoid arthritis (RA) patients and 45 controls consisting of 26 normal subjects and 19 miscellaneous non-RA patients were performed and results were evaluated as relative responses. Low responses (consisting of relative response less than 38%) were found in 31 out of 43 RA patients in cultures against eight of the RA stimulators. The remaining 35 RA stimulators tested yielded only normal mixed lymphocyte culture reactions. The same RA patients used as responders never produced low responses when stimulated by non-RA lymphocytes. But six of the control subjects gave low responses to two RA stimulators. The low responses did not appear to correlate with intake of aspirin, prednisolone, or gold salts, nor could they be reproduced by addition of RA serum of 7S or 19S fractions thereof containing either polyclonal or monoclonal rheumatoid factors. Short-term culture and washing before mixing with the allogeneic cells did not change the low responses suggesting that in vivo bound autoantibodies against lymphocyte receptors were not involved. Study of the inheritance of HLA and mixed lymphocyte culture determinants in the family of patient A. C. who most frequently elicited low responses indicated she was homozygous for a lymphocyte-defined determinant which has been called R. The low responses to A. C. could be interpreted as typing responses based on sharing of the same or of a similar lymphocyte-defined determinant. This gene appears to be increased in patients with RA with respect to non-RA controls and may reflect an association of genes within the HLA chromosomal region leading to predisposition for the development of RA.