Antigen-antibody systems provide the flexibility of varying the kinetics and affinity of molecular interaction and studying the resulting effect on adhesion. In a parallel-plate flow chamber, we measured the extent and rate of adhesion of rat basophilic leukemia cells preincubated with anti-dinitrophenyl IgE clones SPE-7 or H1 26. 82 to dinitrophenyl-coated polyacrylamide gel substrates in a linear shear field. Both of these IgEs bind dinitrophenyl, but H1 26.82 has a 10-fold greater on rate and a 30-fold greater affinity. Adhesion was found to be binary; cells either arrested irreversibly or continued at their unencumbered hydrodynamic velocity. Under identical conditions, more adhesion was seen with the higher affinity (higher on rate) IgE clone. At some shear rates, adhesion was robust with H1 26.82, but negligible with SPE-7. Reduction in receptor number or ligand density reduced the maximum level of adhesion seen at any shear rate, but did not decrease the shear rate at which adhesion was first observed. The spatial pattern of adhesion for both IgE clones is well represented by the first-order kinetic rate constant kad, and we have determined how kad depends on ligand and receptor densities and shear rate. The rate constant kad found with H1 26.82 was approximately fivefold greater than with SPE-7. The dependence of kad on site density and shear rate for SPE-7 is complex: kad increases linearly with antigen site density at low to moderate shear rates, but is insensitive to site density at high shear. kad increases with shear rate at low site density but decreases with shear at high site density. With H1 26.82, the functional dependence of kad with shear rate was similar. Although these data are consistent with the hypothesis that we have sampled both transport and reaction-limited adhesion regimes, they point out deficiencies in current theories describing cell attachment under flow.