Single crystals of the regular 1:1 complex of T and R-state tetramers of L-lactate dehydrogenase from Bifidobacterium longum have been grown from polyethylene glycol 6000 solution in the presence of NADH, fructose 1,6-bisphosphate and oxamate. The crystals belong to space group F222 with unit cell dimensions of a = 148.4 A, b = 295.9 A and c = 71.0 A. The crystals are suitable for X-ray analysis and diffract to beyond 2.5 A spacing. A crystallographic study showed that the asymmetric unit contains two subunits and that one of them belongs to the T-state tetramer and the other to the R-state tetramer.