The expression of human leukocyte function-associated antigens (HLFA, equivalents of murine LFA-1) was studied on early lympho-hemopoietic cells in infant thymus and normal bone marrow by double immunofluorescence methods, cell sorting and colony-forming assays. Monoclonal antibodies MHM-24 recognizing the alpha chain (180 kDa) and 60.3 and 60.1 identifying the beta chain (95 kDa) were used. The vast majority of thymocytes including large terminal deoxynucleotidyl transferase (TdT)-positive blast cells are HLFA positive. On the other hand, most B cell precursors in the bone marrow, identified by the expression of nuclear TdT, do not show surface HLFA which appears at the pre-B cell stage (cytoplasmic mu positive, surface Ig negative). Cell sorting experiments revealed an enrichment of early myeloid cells (myeloblasts and colony-forming unit cells) in the HLFA-negative fraction. Erythroid cells appeared to be completely negative from the burst-forming unit erythrocyte stage onwards. Anti-HLFA-antibodies, capable of blocking T cell and natural killer function, may have therapeutic potential without interfering with precursor cell development in man.