A method is described for the production of mouse monoclonal antibodies to HBsAg. 4 clones produced antibodies directed at the a and one at the d determinant of HBsAg. These antibodies were conjugated to horse radish peroxidase and used in an Elisa for the detection of HBsAg in donated blood. Antibody C10 produced conjugate which rendered the Elisa as sensitive as a commercially obtainable immunoradiometric assay with which it was compared. Conditions for this assay were optimised and it may be used as a rapid (1 h) test for detecting HBsAg-positive blood. It is practicable to read this Elisa by eye instead of photometrically and it can be thus used under field conditions or in emergencies.