Lymphocytes and monocytes with differential and activation antigens were determined by means of the flow cytometry by using monoclonal antibodies in the peripheral blood of patients after organ transplantation. Examinations of the course showed that in this way the acute activity is seized by the cellular immunosystem. Acute reject crises, systemic virus infections, as well as septic states lead to characteristic changes and these verify the diagnostic assertion of this immunomonitoring programme.