Meth A cells block CTL activity in an MHC-unrestricted manner in bystander assays. Shown is a representative experiment with H-2^d anti-H-2^b CTLs. The ⁵¹Cr-labeled target consists of Con A-stimulated H-2^b splenocytes, which in an unlabeled form serves as the control "cold" bystander target. Other cold targets are listed at the bottom of the figure. The diamond indicates CTL killing in the absence of cold target (effector to target ratio of 10).