Spleen cells from mice infected with the virus of Newcastle disease (NVD) fail to mediate the passive transfer of contact sensitivity to simple chemical haptens such as picryl chloride (Pcl) and oxazolone (Ox). The inhibitory effect of NDV can be bypassed by treating recipient mice with low doses of cyclophosphamide (Cy), suggesting that the T-effector cell which mediates the passive transfer of contact sensitivity is not affected by NDV infection. Vicia Villosa-adherent cells from immune mice display contrasuppressor activity and restore the ability of cells from NDV-infected mice to transfer contact sensitivity to naive recipients. In contrast, Vicia Villosa-adherent cells from NDV-infected mice fail to exert any contrasuppressor activity. Furthermore, contrasuppressor activity can also be detected in the culture supernatants of Vicia Villosa-adherent cells from uninfected, sensitized mice, but not in culture supernatants of Vicia Villosa-adherent cells from NDV-infected mice. The present results suggest that a Vicia Villosa-adherent contrasuppressor cell population is impaired by NDV infection.