Haemoglobins from adult sheep and from foetuses of different ages were crystallized, using a standard procedure. The optical properties, cell dimensions and space-groups of the wet and, where possible, of the air-dried crystals were determined, and surveys were made of the lower order diffraction patterns in each of the principal crystal zones. The results obtained provide additional evidence for the non-identity of the foetal and adult haemoglobins and lead to certain conclusions relevant to the general problem of haemoglobin structure. Methaemoglobin from foetuses aged less than 120 days was found to crystallize in either of two forms, each of which was different from methaemoglobin crystals of adult sheep prepared by the same method. Haemoglobin from later foetuses is difficult to crystallize, and its properties resemble those of artificial mixtures of adult and early foetal haemoglobins. No general picture of the structural differences between adult and foetal haemoglobin has emerged from this study, mainly because these were obscured by differences in molecular packing. Indications were found of a difference in molecular symmetry between these two proteins, and this could be correlated with different splitting properties in solution. There is also evidence of a pronounced intramolecular layering in foetal haemoglobin. A new method of controlled shrinkage of protein crystals at constant humidity was developed and applied to foetal haemoglobin crystals, which were shown to pass through a series of at least four well-defined lattices. The densities of wet adult and foetal haemoglobin crystals were measured and used for the estimation of 'bound water'. The amounts of 'bound water' were of the same order as those obtained from haemoglobin crystals of other species.