MY-174, a monoclonal antibody that reacts with specific sialylated O-linked glycoconjugates of chick sialoprotein associated with cones and rods (SPACR), also recognizes another molecule of 300-kDa. Here, we verified that this 300-kDa molecule is chick sialoproteoglycan associated with cones and rods (SPACRCAN), another member of a novel interphotoreceptor matrix molecule family. Screening for chick SPACRCAN was carried out by plaque hybridization using a probe for chick SPACR. Specific polyclonal antibodies raised against chick SPACRCAN were used for the following experiments. To determine whether the 300-kDa molecule detected by MY-174 was identical to the 300-kDa chick SPACRCAN, the migrations of these bands were examined after various glycosidase digestions. Furthermore, the expression levels were measured during retinal development, and compared with those of chick SPACR. The results demonstrated that the 300-kDa molecule recognized by MY-174 was chick SPACRCAN, and we further identified it as a proteoglycan with chondroitin sulfate chains. SPACRCAN possessed heavily sialylated N- and O-linked glycoconjugates, and its MY-174 antigenicity was abolished by O-glycanase treatment after neuraminidase treatment, as observed for chick SPACR. During retinal development, the expression levels of the SPACRCAN mRNA, core-protein and MY-174 antigen as well as its hyaluronan-binding ability peaked around embryonic day 17 and then gradually decreased, whereas all the corresponding SPACR expression levels simply increased except for its hyaluronan-binding ability. The MY-174 reactivity of SPACRCAN in the adult retina was decreased compared to the newborn level, whereas that of SPACR was increased. The decreased hyaluronan-binding of SPACR was induced by an inhibitory effect of the excess of sialic acids in the adult stage. Thus, SPACRCAN and SPACR, with similar core-protein structures and specific sialylated glycoconjugates but distinct chondroitin sulfate chains, may have separate roles in the retina due to their differing expression profiles during development.