Considerations of packing of molecules in various forms assumed by mammalian haemoglobin agree with deductions as to the general overall dimensions of the hydrated molecules based on X-ray diffraction effects and described in a previous paper (Bragg & Perutz, 1952).The following figures for types of haemoglobin crystal listed at the beginning of this paper indicate the degree of consistency between various estimates: Type 1.Either 50×50×75A or 55×55x65/~.Type 12. Same as above.Type 4. 56×56×72A.Type 10. 54 × 54 × 69 A.Though insufficient data are as yet available for deductions from the packing in the other types of molecule listed in this paper, the unit cells and symmetry are not inconsistent with these dimensions except in the case of foetal sheep haemoglobin, which would appear to have a different structure.