Investigation on the ability of nitrogen usage by N_2-fixing Nostoc muscorum and cultured tobacco cells when they were associately cultured on nitrogen-free media was carried out. Also, effect of polyamines on the associated cultured condition was carried out. In addition, morphological changes of N. muscorum and cultured tobacco cells in associate culture were observed to detect the possibility of induction of nitrogen fixing ability on cultured plant cells. The activity of nitrogenase increased markedly when N. muscorum was grown exclusively on nitrogen-free media. When N. muscorum was cultured associately with cultured tobacco cells on nitrogen-free media. When N. muscorum was cultured associately with cultured tobacco cells on nitrogen-free media containing polyamines, high activity was detected in 10^-4 M spermine treated group. Investigation on the change of polyamine amounts showed two times increase in spermidine and eight times increase in spermine on a associate culture. These effects of associated culture were shown through morphological change such as dense loclization of N. muscorum around the cultured tobacco cells as well as inside the cells. These results indicate the viability of N. muscorum in cultured tobacco cells and possible induction of nitrogen fixation ability by symbiosis.