A monomeric Fab'-horseradish peroxidase conjugate was prepared from a small quantity of rabbit affinity-purified anti-human chorionic gonadotropin (hCG) F(ab')2 using normal goat F(ab')2as a carrier. Rabbit affinity-purified anti-hCG F(ab')2(about 0.04 mg) was mixed with 1 mg of normal goat F(ab')2, reduced to Fab' and conjugated to peroxidase using N-succinimidyl 6-(N-maleimido)hexanoate. The rabbit Fab'-peroxidase conjugate was separated from normal goat Fab'-peroxidase conjugate by passage through a rabbit (anti-goat IgG) IgG Sepharose 46 column and shown to be comparable, in both the purity and the ability for a sensitive sandwich enzyme immuncassay of hCG, to that prepared from a sufficient amount of affinity purified anti-hCG F (ab')2without using a carrier.