Anti-mouse renin specific Fab fragment was labelled with 125I and repurified on an affinity-column with renin as ligand in order to eliminate Fab fragments with a damaged antibody combining site. The 125I-labelled Fab fragment, with addition of unlabelled Fab fragment, was administered in vivo to sialoadenectomized mice. The serum disappearance curve of the Fab fragments with the intact antibody combining site possessed two components with a half life of 120 min for the slowest component. The renin specific Fab fragments inhibited the enzymatic activity of renin with a KI-value of 5.5 +/- 1.5 x 10(-9) M, compatible with competitive inhibition. The same Fab fragments were able to inhibit 99% of the enzymatic active renin. The administration of the Fab fragments caused a 3-4 fold increase in the total concentration of plasma renin, but the enzymatic activity was inhibited. No decrease in blood pressure was demonstrated in the animals.