Twenty-one lambs were used to investigate whether their response to an infection with Ehrlichia phagocytophila was dose-dependent. Four groups of four lambs were infected intravenously with a dilution in physiological saline of Ephagocytophila-infected sheep blood containing either 1-3 x 105 infected cells, or approximately 43 infected cells, 4-3 infected cells, or 1-3 infected cells (mean values) and four lambs were left uninfected. The incubation period was significantly shorter in the lambs infected with the highest dose of Ephagocytophi/a. However, the clinical and haematological changes observed, and the weekly weight gains of the lambs were independent of the dose of Ephagocytophila. As little as one Ephagocytophila infected cell may be enough to transmit the infection.