Ten derivatives of 6-aminonicotinamide (6-ANA, I), 6-methylamino-(II), 6-ethyl-amino-(III), 6-isopropylamino-(IV), 6-cyclohexylamino-(V), 6-benzylamino-(VI), 6-dimethylamino-(VII), 6-diethylamino-(VIII), 6-piperidino-(IX), 6-morpholino-(X) and 6-pyrrolidino-nicotinamide (XI) were synthesized, and their biological activities were examined. Similar to I, intraperitoneal administration of II to XI to male mice of ddY strain produced acute toxicity, and the main symptoms were paralysis of hind legs and diarrhea. However, simultaneous administration of nicotinamide prevented appearance of toxic symptoms. LD50 increased in the order of I, IV, VIII, III, II, VI, V, VII, XI, IX, and X. Effect of these compounds on Lactobacillus arabinosus 17-5 (ATCC 8014), which requires nicotinamide was examined. In the incubation medium containing I and IX showed strong and weak inhibitory effect, respectively, on the growth of this micro organism, but other compounds did not show any inhibitory effect, or rather showed additive growth-promoting activity. These compounds, except I, IX, and X, could generplly replace nicotinamide and promoted bacillary growth in the incubation medium which lacked nicotinamide. The substituted derivatives of primary amines showed greater replacing activity than the secondary amine derivatives, and in the former, replacing activity decreased in the order of IV, VI, II, and V. However, this activity was rather small and the strongest IV showed only 0.0003 times as much activity as nicotinamide.