The H-2, thymus-leukemia (TL), and Qa-2 antigens of mice are encoded by closely linked genes on murine chromosome 17, and have structural similarity in that each antigen is borne on a ≈ 44,000 dalton molecule associated with β2 microglobulin (β2µ). The extensive homology of major histocompatibility complex (MHC) products that exists for the mouse and guinea pig suggested that a similar homology might exist for products of genetic regions closely linked to the MHC. By taking advantage of the selective association of β2µ with H-2, Qa-2, and TL antigens, and by using the technique of sequential immunoprecipitation, we demonstrated two previously undescribed guinea pig molecules reactive with anti-guinea pig β2µ. The first molecule was composed of a 36,000 dalton glycoprotein associated with β2µ and was found on guinea pig thymocytes, but not lymphocytes. The second molecule was composed of a 40,000 dalton glycoprotein associated with β2µ, and was found on both guinea pig thymocytes and lymphocytes. By structure, chemical composition, association with β2µ, and tissue distribution, the first molecule is an attractive candidate for the guinea pig homologue of TL antigen, whereas the second fits the criteria for the guinea pig homologue of Qa-2 antigen.