Adenosine deaminase inhibitor was extracted from culture broth of Streptomyces sp. strain V-8 with ethylacetate. The ethylacetate extract showed the characteristic UV absorption spectrum of actinomycins at 440-450 nm. The ethylacetate extract was compared with respect to inhibitory behavior against adenosine deaminase from calf intestinal mucosa with actinomycin D, -C complex and actinomycin V. The Ki values for actnomycin D, -C complex, and actinomycin V against adenosine deaminase were determined to be 9.9 $ imes$ 10$^{-6}$ M, 9.6 $ imes$ 10$^{-6}$ M and 9.3 $ imes$ 10$^{-6}$ M, respectively. The Ki value for the ethylacetate extract of culture broth against adenosine deaminase was determined to be 5.7 $ imes$ 10$^{-6}$ M. The kinetic parameters of actinomycin D, -C complex, -V and ethylacetate extract of culture broth for adenosine deaminase were as follows:I$_{50}$ = 1.5 $ imes$ 10$^{-5}$ M (actinomycin D), 2.7 $ imes$ 10$^{-5}$ M (actinomycin C complex), 3.5 $ imes$ 10$^{-5}$ M (actinomycin V), 8.9 $ imes$ 10$^{-6}$ M (ethylacetate extract of culture broth). The adenosine deaminase was inhibited noncompetitively by ethylacetate extract of culture broth as well as by actinomycin D, -C complex and actinomycin V.