The preparation of a specific anti-T cell serum, applicable in the indirect immunofluorescence technique on cell suspensions, smears of peripheral blood and bone marrow and on tissue sections is described. Rabbits were immunized with thymocytes; after removal of antibodies against species-specific antigens and antigens common to leucocytes by absorptions with red cells and granulocytes, specificity for thymocytes was obtained by repeated absorptions with CLL cells, used as B cell equivalents. Subsequently, the IgG fraction of the absorbed antiserum was isolated. This antibody preparation was tested with various types of blood cells as well as with cell suspensions depleted or enriched in T cells. For the study of tissue sections it had to be absorbed with liver powder. When studying lymphatic tissue it was found to stain only thymus-dependent areas in the specimens tested.