Cell detachment by laminar shear stresses was used to characterize cellular interactions with hydrophilic glass and hydrophobic silane. In this study, we examined whether smaller, rounder cells were preferentially detached by laminar flow, and whether cell detachment occurred by dissociation of adhesion proteins and their membrane receptors or rupture of the membrane. Shear-induced detachment from glass and silane were similar after 0.5 h static attachment to the surfaces, even though 3T3 cells had a greater projected area on silane. No particular cell size was preferentially detached by fluid shear stresses. After 2 h attachment and spreading, 3T3 cells were more easily detached from the silane surface even though the cells were more spread than on glass. On glass, smaller cells were preferentially detached below 30 dyne/cm2, increasing the mean projected area of the population. Above 30 dyne/cm2, larger cells also detached from the surface. Cell detachment from the silane surfaces did not show any size preference. The strength of adhesion and projected areas on both surfaces increased significantly when the surfaces were preincubated with fibronectin. Simple geometric models of spreading cells were used to estimate the forces exerted on cells. The hydrodynamic forces exerted on spreading cells were similar, but the bond density needed to resist detachment declined as the projected area increased. Analysis of Dil-C18(3) membrane fragments indicated that cell detachment by membrane rupture was a significant mechanism of cell detachment from glass for shear stresses above 40 dyne/cm2, but was unimportant for cell detachment from the silane surfaces. The results indicate that differences in the strength of 3T3 cell adhesion were probably due to differences in bond strength and the numbers of receptor-ligand bonds formed on the two surfaces and, on glass, cell detachment due to membrane failure at higher shear stresses.