An attempt was made to purify the active principle (s) of cornea-and muscle-cornin, inhibiting the DNA synthesis in cultured mammalian cells. Each of the two kinds of cornin was separated into the dialysable (D) and the undialysable (U) fraction, which were further fractionated by the use of Sephadex gel. The inhibitory rate of these fractions was indicated by percent of control estimating the incorporation of (3)H-thymidine into cells or DNA fractions of cultured rat liver cells, line RLN-10, during the incubation period of 6 hours at 37℃ in the presence of respective fractions at the final concentration of 0.5 percent in the culture medium. As a result, the highest activity was shown in the D-Ⅱ fraction of muscle-cornin, which corresponded to the elution peak of the inorganic phosphate, and in the D fraction of cornea-cornin, The activity, however, was also shown in several other fractions. These data suggest that the inhibitory effect of cornin on the DNA synthesis in mammalian cells is due to combined effects of several factors present in cornin.