Expression of selected adhesion molecules of the integrin and immunoglobulin family was investigated on CD34+ leukemic cells in 19 AML and 11 ALL cases to evaluate phenotypic differences in adhesive properties of malignant hematopoietic precursor cells in comparison to normal bone marrow CD34+ cells. Of the beta 2-integrin family, CD11a was expressed on > 50% of CD34+ cells in normal bone marrow and almost all leukemias, whereas CD11b and CD11c were not expressed on CD34+ cells in normal bone marrow, but were found on CD34+ blasts in some leukemias of a heterogeneous immunophenotype. Of the beta 1-family, CDw49d (VLA-4) was strongly expressed on normal CD34+ bone marrow cells and on the blasts of all 30 CD34+ leukemic samples, whereas CDw49b (VLA-2) was absent on CD34+ cells in normal bone marrow, but detected on CD34+ cells in a few leukemias which did not constitute a clinical or phenotypic entity according to the FAB classification or immunocytological analysis. The lymphocyte-homing-associated adhesion molecule CD44 (HCAM) and CD58 (LFA-3) were expressed on CD34+ cells in all investigated cases of normal and leukemic bone marrow. ICAM-1 (CD54), the inducible receptor ligand for CD11a/CD18, although present on CD34+ cells in normal bone marrow, was lacking on blast cells of some ALL and AML cases. So far, the variable expression of beta 2-integrins as well as of VLA-2 and of ICAM-1 could indicate distinct differences in cell-cell or cell-matrix adhesion of leukemic cells in ALL and AML patients.