In attempts to produce diffraction-quality crystals of Fabs from two human IgM cryoglobulins (Pot and Yvo), we observed unexpected interconversions in crystal morphologies. The Pot Fab crystallized in two forms when polyethylene glycol (PEG) 6,000 was used as the precipitating agent. Broad, relatively short rods with hexagonal cross-sections were our crystals of choice. However, long, thin rods that were often curvilinear (i.e., "fibril-like" crystals) also appeared in profusion. A third form crystallizing in water had similar but not identical morphology to the fibril-like crystals. In the presence of NaCl, the hexagonal rods were shortened to hexagonal bipyramids and the growth rates of the fibril-like crystals were also greatly reduced. Under the same conditions, we observed an unusual phenomenon, in which hexagonal pyramids grew along the fibril-like crystals, like candy crystals growing on strings. PEG 6,000 was also used to crystallize the Yvo Fab, but the early crystals were highly twinned and imperfectly formed. Among the additives tested to improve the quality of Yvo Fab crystals, 1% (v/v) dimethylsulfoxide (DMSO) provided the most dramatic effect. Initial clusters of small, irregular crystals were replaced with large crystals suitable for X-ray analysis over a period of twelve days.