This study was devised to observe the cytotoxic activity of alcoholic extracts of some Korean medicinal herbs against murine leukemic lymphocyte (P_(388)) in vitro. The growth rate, dose-response and size distribution of P_(388) cells cultured in the medium with or without the each herb extract were determined, and also the effects of incubation time in the same medium were determined. The experimental results obtained are summarized as follows; 1) The doubling time of P38s cells in culture medium without adding any extract was 11 to 12 hours. 2) In the culture medium of Pisa cells, the cytotoxic activity (1 unit) of each extract, Pa-doo, Dae-geuk, kam-soo, won-hwa, La-bok-ja, Sam-neung, A-chool, Dae-Hwang, Hoi-hyang and Do-in was applicable to 0. 076, 0.032, 0. 137, 0.166, 0. 057, 0.206, 0.068, 0.050, 0.829 and 0.9㎎/㎖ of medium, respectively. 3) The inhibitory effect of each extract against the growth of P388 cells was increased according to the increase of concentration on each extract in the culture medium. 4) The numbers of the P388 cells were significantly decreased by incubation with each alcoholic extract for 9 to 12 hours. 5) The pattern of size distribution of P_(388) cells incubated with the each extract was significantly changed after 6 to 9 hours` incubation.