Three hydroxyproline-containing proteins secreted into the medium by human fibroblasts in culture were isolated and characterized. A minor fraction was identical to the collagen monomer. The major fraction was a form of procollagen, which contained, in addition to pro α and α chains, a component estimated to have a molecular weight of 250,000. This component was a dimer of pro α chains joined by disulfide bonds. The third fraction, much lower in hydroxyproline and hydroxylysine content, was of still greater size. Pro α chains were released upon denaturation and reduction, indicating that this fraction may contain pro α chains linked by disulfide bonds to noncollagenous material.