H-2Kk, a transmembrane protein coded for by the mouse major histocompatibility complex, was modified by trypsinization to remove its intracellular segment and was incorporated into lipid monolayers made on an alkylated glass substrate. Cloned cytotoxic T-lymphocytes specific for H-2Kk were incubated on the monolayer. More cytotoxic T-cells bound to monolayers containing the H-2Kk fragment (tH-2Kk) than to control monolayers made without H-2Kk. Preincubation of monolayers with alloantisera against H-2Kk reduced binding of CTL to levels seen with control monolayers. This method offers the possibility of studying specific binding of cytotoxic T-cells to well-defined model membranes.