Assimilatory ferredoxin-nitrite reductase (ammonia: ferredoxin oxidoreductase, EC 1.7.7.1) from the green alga Chlamydomonas reinhardtii has been purified 340-fold with a specific activity of 89.0 U/mg protein. The enzyme has an Mr of 86 000 and is composed for two subunits of Mr 63 000 and 25000, respectively. Nitrite reductase shows absorption maxima at 280, 386 (Soret), 574 (α) and 690 nm, with an ε386 of 2.98 · 104 M−1 · cm−1, and an absorbance ratio, A280/A386, of 1.76. Anaerobic addition of dithionite results in the loss of the 574 and 690 nm peaks, and the appearance of a shoulder at 590 nm. With nitrite, the 690 nm absorption band of substrate-free enzyme disappears and the absorbance in the α region is altered. CO inhibits nitrite reductase and forms a complex with the dithionite-reduced enzyme, with absorption maxima at 554 and 595 nm. These results suggest that ferrodoxin-nitrite reductase from C. reinhardtii contains siroheme as prosthetic group, which is involved in the catalytic reduction of nitrite. Analytical data based on the absorption of the pyridine hemochromogen derivative showed 0.65 molecules of siroheme per enzyme molecule