Humoral and cell-mediated immune responses of sheep infected with Cytoecetes phagocytophila, the causative agent of tick-borne fever, were studied. Previous immunological studies were hampered by lack of reliable sources of antigen. Attempts, therefore, were made to improve antigen yields. Two methods were used successfully. The first was based on the properties of corticosteroids to induce granulocytosis; sheep injected with betamethasone sodium phosphate injection BP at the peak period of parasitaemia had a neutrophilia with an increase of infected cells within six hours. The second method involved the culture of infected blood overnight at 37°C which increased the number of infected cells and the number of organisms per infected cell. A relationship between complement fixing antibodies and pro¬ tective immunity was found and a threshold of protective immunity established. The kinetics of antibody responses to the organism were studied by fractionating immunoglobulins of sera from immune sheep by gel-filtration and ion-exchange chromatography and by complement fixation test. Antibody response to _C. phagocytophila was characterised by an initial production of IgM followed by IgG but the IgM persisted for long periods. The complement fixation test was also used to assess the antigenic relationships between strains of Ch phagocytophila. All strains tested showed strong antigenic relationships but quantitative differentiation was possible. A hitherto undescribed in vitro test for cell-mediated immune response of sheep was developed. The cell-mediated immune response appeared earlier than the humoral immune response.