We have used a phage display shot-gun cloning technique to map the binding domains in two cell surface proteins from animal group C streptococci. The proteins, MAG and ZAG, have affinity for alpha (2)-macroglobulin (alpha (2)M), serum albumin and IgG. In this work, parts of cloned i mag and zag genes were randomly cloned into a phagemid vector, and recombinant phages expressing alpha (2)-M- or albumin-binding activity were isolated through panning against immobilized alpha (2)M or albumin. Analysis of the clones revealed two distinct alpha (2)M-binding sites in protein MAG and two slightly overlapping binding sites in protein ZAG. The minimal albumin-binding domain in protein ZAG, as deduced from the affinity selected clones, consisted of 42 amino acids. These results show that the phage display shot-gun cloning is a rapid and convenient way to characterize the binding site(s) in receptor proteins without any prior knowledge of their number, size, and localization.