In the past, laboratory technics for the diagnosis of the virus and rickettsial infections have been too complicated for the routine hospital laboratory, and these procedures were largely limited to research laboratories with highly trained personnel. Recently serologic tests for many of these diseases have been developed to the point that hospital laboratories equipped to carry out complement fixation tests can perform many of these tests routinely and be of great diagnostic aid to the clinician. Also commercial diagnostic antigens and antiserums are now available for many of the virus and rickettsial diseases. The serologic procedures which are chiefly applicable to routine work include the complement fixation and hemagglutination inhibition tests. These tests depend on the demonstration of a significant rise in antibody titer in serums obtained during the acute and convalescent phases of illness. This paper is a general discussion of the present status of the laboratory diagnosis of