The clearance of natural and recombinant prourokinase (proUK) from the blood of rabbits was studied by means of a double-isotope method which allowed the differential removal of two distinct proUK species to be monitored when simultaneously administered to an individual animal. In initial experiments, proUK expressed in different cell lines contained between 0 and 2.5 molecules of sialic acid per molecule of protein. A slight trend toward slower clearance of proUK with higher sialic acid content was observed but rate differences were not statistically significant. Recombinant proUK produced in CHO cells grown in flow reactors, contained unusually high levels of sialic acid in excess of 3 moles/mole protein. Controlled exposure to immobilized neuraminidase was used to remove sialic acid from this protein in defined amounts. The clearance of the parent material was biphasic with average alpha and beta half-lives of 1.7 min and 16.7 min respectively. The AUC of the parent material was only slightly lowered upon removal of 30% of the original sialic acid. Species with 60% or 90% removal of sialate were much more rapidly cleared from the circulation respectively yielding AUCs equal to 56% and 41% of that observed with the parent material. Thus proUK containing 2.5-3.5 sialic acid molecules per molecule of protein turned over significantly more slowly in rabbits than did less sialylated proUK. The clearance rate was relatively insensitive to sialic acid content between 0 and 1.5 sialic acid residues per proUK molecule.