Cat-301 and VC1.1 are monoclonal antibodies that recognize surface- associated molecules on subsets of mammalian CNS neurons. Earlier work demonstrated that Cat-301 recognizes a 680-kDa chondroitin sulfate proteoglycan (PG). VC1.1 has been shown to recognize 3 polypeptide bands on Western blot analysis; a major band at 95-105 kDa and additional bands at 145 kDa and 170 kDa. In the present report, we show that VC1.1 also reacts with a high-molecular-weight glycoconjugate. Immunoprecipitation experiments and biochemical characterizations indicate that Cat-301 and VC1.1 define at least 3 distinct high- molecular-weight antigens. The VC1.1 antigens react with antikeratan sulfate antibodies, while the Cat-301 antigens do not. By immunodepletion, we show that some VC1.1 antigens are Cat-301 positive, while others are Cat-301 negative. In addition, Cat-301-reactive proteoglycans are heterogeneous with respect to the presence or absence of VC1.1 epitopes. Double-label immunofluorescence studies with these 2 antibodies are consistent with the biochemical results and show that there are 3 classes of immunoreactive neurons in the cat CNS:Cat- 301+/VC1.1+, Cat-301-/VC1.1+, and Cat-301+/VC1.1-. These results indicate that structural microheterogeneity exists among Cat-301 and VC1.1 high-molecular-weight glycoconjugates. This heterogeneity may be a reflection of the diverse neuronal phenotypes that are recognized by Cat-301 and VC1.1 in the mammalian CNS.