Previous studies have shown that thrombocytopenia reduces the lodgement of circulating tumor cells, thus indicating a role of platelets in tumor cell lodgement. In this study, a vital fluorescence microscopic technique, electron microscopy and isotope measurements were combined to analyse the very first phase (5 min) of tumor cell lodgement using a syngeneic, methylcholantrene induced fibrosarcoma. The mode of tumor cell arrest in the liver microvasculature of normal and thrombocytopenic rats was studied. In addition, tumor cell size and deformability were investigated in vitro. The in vitro experiments showed that the fibrosarcoma cells were very rigid. Both the vital microscopic and electron microscopic studies indicated that the tumor cells mainly became arrested by mechanical trapping in narrow liver sinusoids. Arrest of tumor cells by adherence to venular walls was a rare finding. Platelets did not seem to influence any of these initial phenomena of tumor cell lodgement. The isotope measurements, which showed no difference in the number of tumor cells lodged in the liver between normal and thrombocytopenic rats, further indicated that the initial arrest of the fibrosarcoma cells was not influenced by platelets.