We have cloned and characterized cDNAs encoding the alpha 1 chain of type XV collagen from a human placenta library. Using primer extension cloning we extended the cDNAs to the 5' end of the mRNA and determined the complete deduced primary structure of the human alpha 1(XV) chain. The polypeptide chain contains nine triple helical domains separated by eight non-triple helical regions and flanked by large amino-terminal (555 amino acid residues) and carboxyl-terminal (256 amino acid residues) non-triple helical domains. Comparison of amino acid sequences of the human alpha 1(XV) chain with those of mouse alpha 1(XVIII) collagen showed remarkable similarity within both amino- and carboxyl-terminal non-triple helical domains. Within the carboxyl third of the amino-terminal domain a tandem repeat structure is found with an about 45-amino acid residue sequence repeated four times. This amino acid sequence has a strikingly high similarity to rat cartilage proteoglycan core protein. Northern blot analysis of human embryonic RNA revealed that alpha 1(XV) mRNA is expressed predominantly in internal organs such as the adrenal gland, kidney, and pancreas.