An experimental system for the possibility of association between cultured plant cells and a cyanobacterium (N. muscorum) was investigated. The association was established in nitrogen-free medium, and nitrogen source was provided by N. muscorum through its own nitrogen fixation. N. muscorum was distributed on the outer part and intercellular spaces of the cultured plant cells. Polyamines were treated to the nitrogen-free medium to improve the association. Polyamines increased nitrogenase activity and protein amount in the association between N. muscorum and the cultured plant cells, as compared to the association without polyamines. Especially, spermine showed an increasing effect on the nitrogenase activity and the protein content in the association of tobacco and N. muscorum, while spermidine showed similar effect in case of soybean and N. muscorum.