In this study we compared the effect of structural stresses and surface roughness on biocompatibility of NiTi- and Ti-alloy for ROS-17/2.8 osteoblastic cells. We suggest here that cell viability and cell attachment are linear functions of internal (structural) stress and subgrain size of the implant alloy. However, this is not the case with surface roughness. The two-phase state in these materials is characterized by different mean values of structural stresses (sigma) in alpha-martensite and beta-phase. We found a straight correlation between cell viability and sigma(beta)/sigma(alpha) ratio. Atomic force microscopy revealed that, even after equal surface polishing treatments, roughness varied significantly between the different alloys. The effect of the surface structure of the alloy on the osteoblastic ROS-17/2.8 cell survival rate was studied with combined calcein-ethidium-homodimer fluorescence labeling. The possible effects on cell attachment to substrate were studied by staining the focal contacts with paxillin antibody. All the NiTi surfaces were tolerated well and the cells attached most abundantly to the roughest NiTi surface but the smoothest Ti-alloy surface. However, other parameters of the material state, such as the surface stresses created by hot rolling seem to be responsible for some of the attachment and cell survival features observed in this study.