A growing number of differentiation antigens of murine Tand B-cells has been described, including the Ly series of lymphocyte alloantigens. Some of these have been partially characterized by biochemical means (Durda and Gottlieb 1976, 1978, Durda et al. 1978, Cone et al. 1978, Tung et al. 1977, Kessler et al. 1978). We report here some molecular properties of the two known alleles of another Ly antigen, Ly-5. The Ly-5 antigen, as described by Komuro and co-workers (1975) is restricted to thymocytes and thymus-derived cells. It is present on both killer and helper cells (Woody et al. 1977, McKenzie, unpublished data). Most of the common inbred strains, including C57BL/6, BALB/c, CBA, A/J, and DBA/2, carry the Ly-5.1 allele. The Ly-5.2 allele, on the other hand, has been found only in SJL, STS, and DA strains (Komuro et al. 1975, McKenzie and Potter 1979). In this study, thymocytes from C57BL/6, BALB/c, and SJL/J were biosynthetically labeled with 3H-amino acids and the cell extracts partially purified by chromatography over a column of Lens culinaris lentil lectin-Sepharose (see legend to Figure 1 for details). This procedure selectively isolates glycosylated molecules due to the binding of mannose to the lectin, indirect immunoprecipitations of the lentil-lectin purified extracts were performed using antisera specific for Ly-5.1, ([SJL/J x DA]F 1 anti-A.SW) or Ly-5.2 (A.SW x SWR)F 1 anti-SJL/J) followed by Staphylococcus aureus (McMillan et al. 1977, Cullen-and Schwartz 1976). Antigenantibody complexes eluted from the Staphylococcus were resolved on 10% SDS polyacrylamide gels run under reducing conditions. Figure 1 shows gel profiles obtained by immunoprecipitation with the anti-Ly sera as well as with anti-/? 2 microglobulin. A peak running at about 130 000 daltons is obtained using anti-Ly5.1 sera on extracts of C57BL/6 and BALB/c (Ly-5.1 strains), but not on SJL/J (aLy-5.2 strain). On the other hand, SJL/J extracts react with anti-Ly-5.2 to produce a peak of 130 000 daltons, whereas the B6 and BALB/c extracts are negative with this antiserum. Similar peaks could be immunoprecipitated from spleen and lymph node cells (data not shown). All of the immunoprecipitations have a nonspecific