During combined nitrogen step-down, filaments of cyanobacterium Anabaena sp. PCC 7120 differentiate about 5-10% of vegetative photosynthetic cells into heterocysts, the specialized cells for N$_2$ fixation (Walk, 1996). Heterocysts have a thick cell wall reducing permeation of O$_2$ and consist of two additional layers composed of glycolipids and polysaccharides. The difference in structure and composition of the cell wall between heterocysts and vegetative cells allows separation and isolation of heterocyst. Heterocysts isolated by this protocol can be subjected to protein analysis and activity measurements, which do not require strict anaerobic conditions.