The effects of highly purified components of Bordetella pertussis, that is pertussis toxin (PT) and filamentous haemagglutinin (FHA), and of lipopolysaccharide (LPS) were studied in the active mouse weight gain test (MWGT). The PT when given alone or with other components in various combinations caused weight losses and deaths 2-3 days after inoculation but FHA was not toxic in the MWGT. When FHA was given with PT, the toxic effect of PT was reduced. The LPS caused weight losses at 24 h which decreased when LPS was given with PT. The toxic effects of PT as indicated by late deaths and late weight losses or failure to gain weight continued until 14 days after inoculation. The various components had similar effects on mouse weight gain in both LACA and NIH strains of mice. The doses of PT used in the MWGT caused marked leucocytosis but FHA and LPS did not. No agglutinins appeared in the sera of mice inoculated with various purified components. The components were thus pure and did not contain agglutinogens.