The Fab′ fragment of McPC603 mouse myeloma immunoglobulin with phosphorylcholine-binding activity crystallizes into both a hexagonal and a cubic structure. Models of these structures showing approximate molecular packing were deduced from an electron microscopic examination of stained crystal sections, a preliminary X-ray diffraction study, and the knowledge that both crystal structures grow simultaneously in the same crystallizing solution. When both of these models were modified slightly to agree with the more exact molecular packing in the hexagonal crystal from a 4.5 resolution X-ray study, they were both still compatible with the EM data.