Data were presented to indicate that the antimalarial drug chloroquine does not interfere with the normal antibody response in rabbits nor does it depress the levels of formed antibody. Evidence was presented to show that it can interfere to some extent with an antigen-antibody reaction of the type not requiring complement, the fluorescent treponemal antibody (FTA-200) test. The drug inhibited biological complement activity in vitro, and the action must be attributable to a mechanism other than binding of Ca++ and Mg++. Addition of an excess ion served to heighten the complement inhibition in the presence of drug. Chloroquine's therapeutic effect in certain connective tissue diseases may be due to its interference with in vivo complement dependent antigen-antibody reactions occurring in these diseases by inhibiting complement activity.