Antibodies to soluble calf or rat skin collagen were purified by appropriate immunoadsorption and separated into three subfractions directed either to antigenic sites on unfolded alpha-chains (denatured collagen), to nonhelical sites, or to helical sites exposed on the triple helical molecule. In indirect immunofluorescence tests each of the antibody solutions reacted with collagen of skin and kidney tissue, although the latter two antibody solutions appeared to be more active. Distinct activity was also observed with antibodies to the N-terminal antigenic determinant of rat skin collagen alpha2-chain, a structure usually involved in cross-linking. Indirect immunofluorescence tests with anti-collagen sera on sections of skin resulted in the staining of the whole dermis, while anti-procollagen sera revealed binding only to the uppermost subepithelial layer of the dermis (stratum papillare). On kidney sections only the interstitial connective tissue reacted with purified anti-collagen or anti-procollagen sera. Both skin and glomerular basement membranes remained unstained with either kind of purified antibodies. However, antisera not subjected to immunoadsorption do react with the glomerular basement membrane. Antibodies to noncollagenous contaminants are considered to be responsible for this finding which emphasizes the necessity to use purified antibodies exclusively for this type of immunofluorescence analysis.