Ruminant haemoglobin (Hb) extracted from river buffalo (Bubalus bubalis) has been purified and crystallized. Two different Hb forms of the phenotype BB gave isomorphous crystals which diffracted to 2.8 angstrom resolution and were not sensitive to radiation damage. Crystals of CO Hb have space group P2(1)2(1)2(1) with unit-cell parameters a = 54.8, b = 64.0, c = 158.6 angstrom, and contain one Hb molecule per asymmetric unit.