A 170 kD protein, prominent in soluble extracts of rooster arteriosclerotic plaques, has been partially characterized. The protein was eluted from a size exclusion column in a broad molecular weight fraction > 100 kD. Concanavalin A and a murine polyclonal antibody raised against the isolated 170 kD protein reacted with the protein on Western blots. The 170 kD protein had an isoelectric point of approximately 5.4 and was digested by collagenase treatment. Amino acid analysis of a 70 kD fragment of the protein closely resembled that for chick collagen alpha 3 (VI). A 13 amino acid sequence within this 70 kD fragment had 69% identity and 85% homology to chicken collagen alpha 3 (VI). Soluble protein extracts from cultured plaque smooth muscle cells (SMC), and from healthy artery SMC, contained low levels of the protein. These cellular extracts also reacted with the polyclonal antibody described above. Although the protein lacks absolute amino acid sequence identity with collagen alpha 3 (VI) it shares with it many biochemical features, suggesting that the 170 kD protein is a variant species of chick collagen alpha 3 (VI).