Intestinal inflammation occurs in both nematode infections and graft-versus-host disease (GVHD). In nematode infections, this involves the proliferation of mucosal mast cells (MMC) and goblet cells (GC). To examine MMC and GC responses in GVHD, female Lewis rats were given allogeneic bone marrow (BM). Each animal received 1,020 rads and, one day later, 6 X 10(7) ACI BM cells plus 2 X 10(7) ACI spleen cells i.v. Control rats received 6 X 10(7) BM cells. On days 4, 8, 12, 16 and 20 post-transplant, rats were sacrificed and their intestines removed and prepared for histological examination of MMC and GC. Cells in 10 villus-crypt units (VCU) of the gut were counted for each animal. Skin and tongue were also removed and examined to determine the degree of GVHD. GVHD was first evident on day 8 in allogeneic BM recipients and progressed thereafter. No evidence of GVHD was seen in syngeneic BM recipients. Rats receiving allogeneic BM showed a 10-fold increase from day 12 to day 20 (p less than 0.01). Rats receiving syngeneic BM showed no significant change in MMC through the 20th day. In animals with GVHD, GC decreased by day 12 and remained lower than control animals during the subsequent 8 days. It was concluded that, similar to nematode infection, MMC proliferation is a feature of GVHD. In contrast, GC do not appear to proliferate in an acute GVHD.