Two triple-helical domains, Col 1 and Col 2, were obtained from a pepsin digest of human placental collagen XIV and separated from each other under nondenaturing conditions. Edman degradation demonstrated 106 amino acids residues in the Col 1 and 149 residues in the Col 2 domain. All except one of the 37 prolines in the Yaa position of the Gly-Xaa-Yaa triplets were completely hydroxylated to 4-hydroxyproline, and there were three imperfections in the triplet repeat. Partial or complete hydroxylation and glycosylation were found for all seven lysines in the Yaa position. Domain Col 1 was joined by disulfide bonds into a trimer, while Col 2 appeared as a mixture of monomers and disulfide-linked dimers. Circular dichroic spectra were typical for the collagen triple helix and revealed relatively high melting temperatures for Col 1 (38 degrees C) and Col 2 (43 degrees C). An almost perfect refolding of the triple helix was observed for Col 1 but not for Col 2, emphasizing the importance of disulfide bonds for the folding kinetics and in part the stability of the triple helix. Circular dichroic spectra of the large nontriple helical domain, NC3, of collagen XIV indicated 11% alpha helix and 63% beta structure. Comparative melting profiles of NC3 and intact collagen XIV indicated that the triple helices in intact collagen XIV have a melting temperature of 44 degrees C.