The antibody-induced capping of several cell surface components has been investigated by immunofluorescence methods using two mouse cell lines, a parental C58 thymoma line and a mutant derived from it lacking TL and H-2 antigens. Other cell surface components were present in approximately equal amounts on the two cells. Parental cells treated with rabbit antibodies to T200, a major surface glycoprotein, rapidly formed caps containing T200, but the mutant cells similarly treated showed a uniform surface distribution of T200. On the other hand, with a secondary antibody treatment, the T200 on both cells capped equally well. When the indirect T200 caps were examined using a second immunofluorescent stain for H-2, TL, or Thy-1 antigens, it was found that on parental cells all three of these antigens were co-capped with T200; on mutant cells no staining was found for H-2 or TL, as expected, and essentially uniform distribution of Thy-1 was observed. The co-capping of H-2, TL, and Thy-1 antigens with T200 on the parent cell is remarkable, because the first three components are known to be molecularly independent in lymphocyte cell surfaces. The indirect capping of the viral glycoprotein gp 69/71 similarly induced a co-capping of H-2 and TL antigens on the parent cell. These results demonstrate that H-2 and related molecules may co-cap with a variety of independent cell surface antigens. Such co-capping of histocompatibility components could play an important role in a proposed dual recognition mechanism for cell-mediated cytotoxicity reactions and other immunologically important cell-cell interactions.