||This study examines spatial regulation of some signaling molecules, either housed in or recruited to lipid rafts, during T cell activation. We investigated the specific interactions between antigen presenting cells and CD4 + T cells that result in coalescence of lipid rafts. A novel immunoelectron microscopic approach was used to visualize detergent resistant lipid rafts (DRLRs) which involves coating a primary antibody on nickel grids followed by the capture of DRLRs and subsequent labeling with an antibody to a second signaling molecule and colloidal gold. This assay provides a technique to examine 2 distinct proteins within a single raft. We show heterogeneity among DRLRs isolated from unstimulated CD4 + T cells, with respect to the proteins, Thy-l, Ly-6A.2, and CD3. Initial interactions between the adhesion molecules LFA-1 and ICAM-1 on T cells and antigen presenting cells, respectively, results in membrane reorganization independent of the TCR-MHC class II or TCR-MHC class II-peptide interactions.