Regression of EB‐virus‐induced transformation occurs exclusively in cultures of leukocytes from seropositive donors. Studies have shown that the strength of regression could be assayed in terms of the proportion of T cells which must be added to the autologous EB virus‐infected T‐cell‐depleted population in order to establish regression in the culture. The in vitro regression phenomenon was strongly T‐cell‐dependent but did not require the presence of either monocytes or, as a potential antigenic stimulus, the residual viral envelope material on the surface of virus‐infected B cells. The T‐cell‐depleted population from seropositive donors sometimes transformed (7/60 cultures) without the experimental addition of virus. Regression appeared to be independent of cytotoxic mechanisms involving anti‐viral antibodies and not to be mediated by soluble factors released into culture medium.