The role of CD 54 in the homotypic interaction of normal monocytes and the blasts of five cases of acute myeloid leukemia (AML) was analyzed by immunoelectron microscopy (IEM). The cells were seeded on glass coverslips precoated with an electrontransparent melamine resin, which allowed their in situ labeling with monoclonal antibodies (MoAb) and subsequent analysis by whole mount immunoelectron microscopy (WM-IEM) or transmission immunoelectron microscopy (TIEM). Timed incubation of the cells in serum-free medium +/- interferon-gamma (IFN-gamma, 500 U/ml) induced a spreading of the monocytes and the blasts of four out of five leukemias, characterized by the development of numerous filopodia which led to initial cell-cell contacts. In parallel, an increase in CD 54 surface density in four out of five leukemias could be detected, while no evidence of a CD 54 redistribution (capping) on single cells could be observed. WM-IEM studies detected no CD 54 molecules in the "early" cell-cell contacts, while "later" cell-cell contacts displayed strong CD 54 positivity. These data indicate that CD 54 is not involved in initial cell-cell contacts but is shifted secondarily to the cell contact sides and may thereby stabilize the adjacent membrane areas. The absence of spreading of the CD 54 negative blasts in one out of five leukemias and the blockade of the cellular migration by an anti-CD 54 MoAb (Clone 84H10) in the remaining cases suggest that CD 54 expression is necessary for cellular locomotion. The observed inhibitory effect of the anti-CD 54 MoAb probably mimics a negative circuit that serves to control cellular migration.