Two mouse monoclonal antibodies to chicken immunoglobulin VH-associated idiotypes (Id), CId-1 and CId-2, were used as probes for Id determinants on mouse T cells. CId-1, which recognized chicken antibodies to N-acetyl glucosamine (NAGA), and approximately 0.4% of chicken T lymphocytes also reacted with approximately 0.2% of BALB/c splenic Thy-1.2+ cells. When enriched CId-1+ splenic T cells from NAGA-immune BALB/c mice were fused with the AKR thymoma BW 5147 cell line, 2 of 72 resulting hybrids, termed CId-1A and CId-1B, were reactive by indirect immunofluorescence with the CId-1 antibody. CId-1 determinants were expressed both in the cytoplasm and on the cell surface. Immunofluorescence studies revealed that both CId-1+ T cell hybrids were phenotypically identical: CId-2-/Ig-/Lyt-1+2-/Thy-1.2+/II-2d+/I-Ad-/I-Ak-/I-Jd+/I-Jk+. Incubation of CId-1B hybrid cells with concanavalin A or lentil lectin resulted in capping of the CId-1 determinant, whereas incubation with pokeweed mitogen, lipopolysaccharide, phytohemagglutinin, and wheat germ agglutinin had no effect on the cell surface distribution of the CId-1 molecule. Trypsin or pronase treatment resulted in the loss of detectable CId-1 determinant on the cell surface. Treatment of CId-1B cells with tunicamycin also reduced the immunofluorescence intensity of the surface CId-1 determinant, but had no effect on its cytoplasmic expression. CId-1 antibody-induced capping of the CId-1 marker did not affect the surface distribution of Lyt-1, Thy-1.2, H-2d, I-Jd, or I-Jk molecules. Conversely, capping of I-Jd and I-Jk determinants did not alter the surface distribution of CId-1. These results suggest that the CId-1 determinant is on a glycoprotein that is not physically linked to the Lyt-1, Thy-1.2, H-2d, I-Jd, and I-Jk molecules. The clonal restriction of CId-1 expression by T cells suggests that the CId-1+ molecule could be a T cell antigen receptor.