Vital microscopic studies on tumour cell lodgement in the rat liver have suggested that tumour cells from culture (CTCs) and from solid tumour (STCs) have different rheological properties. In the present study CTCs and STCs from a rat fibrosarcoma were compared in respect to their morphology, rheology and lodgement properties. The ultrastructural examination showed that the CTCs had a larger mean diameter (14.9μm) than the STCs (11.2μm). Both cell types had a very irregular nucleus. Surface irregularities provide the CTCs and STCs with a ‘membrane excess’, i.e. a larger plasmalemma than required to enclose the cells as smooth spheres, amounting to 46.8% and 60.9%, respectively. These values indicate that both CTCs and STCs can be substantially deformed with preservation of the volume and area. The CTCs were stiffer than the STCs when deformed at constant pressure in 6.5μm glass pipettes, the nucleus appearing to be a significant hindrance to CTC deformation. Five minutes after intraportal injection of radiolabelled CTCs and STCs a much higher percentage of CTCs (82%) than of STCs (20%) remained lodged in the rat liver. The results indicate that the propagation in culture of fibrosarcoma cells alters the morphology and rheology of the cells such that CTCs are not suited for studiesin vivo where the spontaneous intravascular dissemination and organ lodgement of tumour cells is simulated.