The turnover of various recombinant single chain urinary plasminogen activator (scu-PA) preparations produced from different host cells were studied in rabbits by means of a double label technique. One scu-PA preparation was labelled with 125I whereas other scu-PA preparations were labelled with 131I. In each experiment 125I-labelled scu-PA was injected concomitantly with a 131I-labelled scu-PA preparation intravenously into each of six rabbits. Blood samples were obtained from the femoral vein and the radioactivity of both labels was quantitated. Recombinant scu-PA preparations included material grown in CHO cells, E Coli, or mouse melanoma cells. The clearance of the different preparations seemed to indicate a dependence on the relative sialylation of the molecule. To further test this hypothesis, CHO-derived scu-PA was desialated with neuraminidase for varying time periods. Three preparations were characterized as being 30%, 60% and 90% desialated. The clearances was directly proportional to the residual sialic acid content of the molecule.