Inhibition of cytolytic activity by Meth A cells requires cell-cell contact. H-2^d anti-H-2^b CTLs, generated as described in Materials and Methods, were tested in a transwell plate using a modification of a cold target inhibition assay. The ⁵¹Cr-labeled target consisted of EL-4 (H-2^b) cells; cold targets are listed across the top of the figure. Cells were arranged in the compartments as indicated in the diagram at the bottom of the figure. In one experiment 50 µl of K44 (10 µg/ml), an MHC class I blocking antibody, was added to each well of the underlying compartment to demonstrate that soluble molecules pass through the transwell membrane. The diamond indicates CTL killing in the absence of cold target (effector to target ratio 10).