Abstract: Heterocysts with higher nitrogen-fixing activity were isolated from Anabaena 7120. The absorbance spectra of their soluble and membrane fractions are different obviously from that of the vegetative cells. The polypeptide compositions in soluble and membrane fractions of these two types of cells were compared by means of SDS-polyacrylamide gel electrophoresis. The results show that about one half of the soluble proteins in vegetative cell are degraded after the heterocyst is differentiated. Heterocyst has polypeptides as those in vegetative cell but it also synthesizes some new polypeptides.The soluble proteins of heterocyst show five major polypeptide bands with apparent molecular weights of 73000, 54000, 48000, 41000 and 34000. In the membrane proteins of heterocyst, at least two polypeptide bands having apparent molecular weights of 41000 and 35000 are deficient in membrane proteins of vegetative cellThe process of heterocyst development is correlated with the fluctuation of intracellular protease activity. Thus, the activity increases rapidly after differentiation initiation and reduces to its original level when the process ceases. Phenylmethylsulfonylfluoride was found to block the formation of heterocyst, whereas rifampicin and casein hydrolysate may double the differentiated frequency of the heterocyst. In the latter situation, two polypeptide bands with apparent molecular weights of 72000 and 56000 in soluble proteins of vegetative cell are deeply stained. The possibility of using the stained bands as a biochemical marker is discussed in this paper.