2-Nitrosoureido-1, 3, 4-thiadiazoles were effective against both mouse lymphoid leukemia L1210 and rat ascites hepatoma AH13. Some 2-acylamino-1, 3, 4-thiadiazoles were also effective against L1210. The anti-L1210 action of these 2-acylamino-1, 3, 4-thiadiazoles was blocked by administration of nicotinamide. Among these compounds, 2-propanoylamino-1, 3, 4-thiadiazole was the most effective against L1210, with a maximum T/C% of 166.