8-chloroadenosine showed marked activity against mice solid tumor hepatoma 22 (H22) and ascitic leukemia L-1210. At 100mg. Kg-1. /d x 7, the inhibition rate of H22 was 71.7 +/- 13.3% (P < 0.01) and 66.1 +/- 4.46% (P < 0.01), i.p. and i.v., respectively; at the same dose, the life-prolonging rate of mice bearing L-1210 was 124.0 +/- 22.1% (P < 0.01) and 104.2 +/- 20.1% (P < 0.01), i.p. and i.v., respectively. 8-chloroadenosine also showed activity against 3 human cancer cell lines in vitro. The IC50 values were determined by measuring cell growth using trypan blue dye exclusion. The results showed that HL-60 and K562, and human gastric cancer cell line MGc80-3 and IC50 values of 1. 8 mumol/L, 4.2 mumol/L and 1.56 mumol/L, respectively. The toxicity of 8-chloroadenosine was low, with LD50 of 1025.0 +/- 52.4 mg/kg for mice and 793.4 +/- 70.1 mg/kg for rats by single i.p. injection.