Male chicks were injected intramuscularly with 6 mg of cyclophosphamide daily for three consecutive days, the initial dose being given on the day of hatching. This treatment impaired bursal development and lymphopoiesis and no circulating antibodies appeared after such chicks were injected with Salmonella gallinarum antigens at 19 and again at 26 days of age. In impaired chicks which were infected orally when 15 days old with S gallinarum the level of mortality was essentially the same as that in unimpaired animals. Impairment did not affect the lower mortality that accompanies feeding with a nutritionally adequate diet containing meat meal as the sole protein supplement in comparison with fish meal as the supplement. An anaemia was present in chicks examined seven days after infection. This anaemia was microcytic to a degree that either exceeded or approached statistical significance. As antibody-mediated hypersensitivity reactions involving the erythrocytes were absent in the impaired chicks, the anaemia must have differed in pathogenesis from the macrocytic anaemia of immunopathological orgin reported by others in adult females with acute fowl typhoid. A disturbance of iron metabolism could have accounted for the anaemia in the diseased chicks.