Blocking of cytotoxic activity of CD4⁺ clone 13 against target cells no. 4 (Table 1) by MoAbs. ⁵¹Cr-labeled target cells were incubated with a saturating concentration of MoAbs for 60 minutes at 37°C. Effector clones were added at an E:T ratio of 2:1 and a standard 4-hour ⁵¹Cr release assay was performed. The percentage of specific lysis is indicated.