The humanized antibody CAMPATH-1 H has been shown in pilot studies to be beneficial in the treatment of lymphoid malignancy and other lymphoproliferative diseases. The antigen recognized by this antibody is not confined to lymphoid cells, and work with rat antibodies of similar specificity has not eliminated the possibility of damage to human hematopoietic progenitors, particularly those capable of repopulating bone marrow and sustaining hematopoiesis. This study aimed to discover if hematopoietic progenitor cells were affected by treatment with CAMPATH-1H, with or without human complement. Bone marrow mononuclear cells from healthy volunteers were treated with saturating concentrations of CAMPATH-1H, human complement, or CAMPATH-1H plus human complement. The CD34-positive fraction of the mononuclear cells was treated similarly. Residual progenitor activity was measured in the colony-forming unit-granulocyte, erythroid, monocyte, megakaryocyte assay and compared with untreated controls. There was no significant difference (at the 5% level) between treated and control cells. Mononuclear cells were divided into CAMPATH-1 H-positive and CAMPATH-IH-negative fractions by fluorescein isothio-cyanate-CAMPATH-1H labeling and fluorescence-activated cell sorter separation. Hematopoietic progenitors were predominantly found in the CAMPATH-1H-negative fraction. Furthermore, mononuclear cells treated with CAMPATH-1 H and complement were equivalent to controls in experiments that investigated the capacity of these cells to form hematopoietic foci in long-term cultures.