Lym-1, an anti-MHC class II Ab, displayed a unique processing pathway after binding to the surface of Raji B-lymphoma cells, in which Fab-like fragments were gradually released into the medium. The fragments had reduced interchain disulfide bonds. Fragmentation was markedly reduced by inhibitors of intracellular catabolism, namely ammonium chloride, chloroquine and leupeptin. The capacity of the process was high, and fragmentation of approximately 5x10(6) Ab molecules per cell per day was measured directly, in what can be considered to be a minimum estimate. Five other Abs to the MHC class II antigen were tested similarly on Raji and on three other B-cell lymphomas: none showed the same high level of fragmentation seen with Lym-1 binding to Raji, but significant fragmentation did occur with some of the Abs, particularly EDU-1 and L243. The level of fragmentation depended on the cell line as well as on the particular Ab. The other 5 Abs were all catabolized, to low molecular weight material, much more extensively than Lym-1. Part of the difference between Abs can probably be attributed to the fortuitous, preferential labeling of Lym-1 on the light chain, since the data suggest that the Fc fragment is fully degraded while the Fab-like fragment is released into the supernatant. This pathway of Ab processing is likely to be related to the physiology of the MHC class II antigen, which recycles into a mildly proteolytic intracellular compartment.