We investigated two cytotoxic monoclonal antibodies of BL-series (BL-IIIB4 and BL-IIG2) according to T-lymphocyte depletion from bone marrow. Both antibodies work together with human complement similar Campath-1, which was tested parallelly. The extent of T-cell depletion is about 95% for all three antibodies. On the other hand the haemopoietic side effects tested by CFU-GM recovery and LTBMC are for the BL-antibodies not as strong as for Campath-1, especially in view of LTBMC. T-cell regeneration could be shown in long term cultures. Our results indicate a possible suitability of the two investigated antibodies for T-cell depletion of bone marrow.