Previous work (HELLER, Nature. Submitted for publication.) had shown that actinomycin D enhanced the growth of chikungunya virus in chick embryo cells and inhibited interferon production. In the present study, chikungunya virus was found to be inactivated at 35 C. at a rate of 1 log in 3 hours. The virus after 23 hours at 35 C. no longer produced plaques but gave good yields of interferon in chick cells. This inactivated virus was compared with stock virus in cells which had been treated with 0.06 (gm./ml. of actinomycin D for 4 hours at 35 C. Actinomycin was found to inhibit interferon production by both although both produced interferon in control cells. No evidence was found of production of an inhibitor of interferon action in cultures treated with actinomycin. The enhancement of virus growth by actinomycin does not therefore appear to contribute to its action in inhibiting interferon. This is the first evidence of interferon production by an inactive arbovirus. CE Gordon Smith.