The present study was undertaken to determine whether a relationship could be found between Fab' antibodies and the occurrence of C-reactive protein, rheumatoid and anticomplementary factors in the serum of patients with malignancies and other illnesses. The findings argue against the possibility that Fab' antibody activity is associated with a single disease entity. The very frequent occurrence of Fab' antibodies in hospital patients as opposed to healthy blood donors implies that their function is probably indirectly related to other immunological events. There is evidence to suggest that Fab' antibodies may serve as a mechanism for clearing immune complexes.