To investigate whether hepatic fibrosis induced by porcine serum in rats is caused by an immune reaction to porcine serum, rats that were immunologically tolerant exclusively to porcine serum were subjected to the repeated injection of porcine serum over a long period. This porcine serum-tolerant group consisted of 15 Wistar rats that had been injected intraperitoneally with porcine serum twice a week from the first postnatal day for 18 weeks. The control group consisted of 16 Wistar rats, aged 8 weeks, that were injected intraperitoneally with porcine serum twice a week for 10 weeks. Livers were fixed and examined by light microscopy. The serum of each rat was subjected to indirect enzyme-linked immunosorbent assay (ELISA) to measure the level of antibody to porcine albumin. In addition, immunohistochemical staining for ED1 was performed on untreated normal and porcine serum-induced fibrotic rat livers to examine the distribution of macrophages and their precursors, the monocytes. All rats in the tolerant group showed an extremely low antibody level (x = 68.27 +/- 4.53), and none (0/15) developed hepatic fibrosis. The majority of rats in the control group showed a very high antibody level (x = 1242.19 +/- 201.15); 75 percent (12/16) developed hepatic fibrosis. Data indicate that, despite the prolonged, repeated injections of porcine serum, if an immune response to porcine serum does not occur, the rats do not develop hepatic fibrosis. The porcine serum-tolerant rats developed hepatic fibrosis after 4 weeks of CCl4 treatment, indicating that injection of porcine serum into neonatal rats did not cause anergy of fibrogenesis, thereby preventing the animal from developing hepatic fibrosis. In normal rat liver, ED1-positive cells, which include nearly all Kupffer cells, were located pre-dominantly in the periportal area. In fibrotic rat liver, ED1-positive cells aggregated prominently in the newly formed and advanced connective tissue septa developed mainly between the neighboring central veins, and in fibrotic parts of the liver capsule. Aggregation of ED1-positive cells was rarely observed in nonfibrotic parts of the liver capsule. The difference between normal and fibrotic rat liver in distribution of EDl-positive cells suggests an involvement of macrophages in fibrogenesis and septum formation. In conclusion, our study showed a significant contribution by the immune response to porcine serum antigens leading to porcine serum-induced rat hepatic fibrosis--processes in which macrophages may be important. This study may lead to an understanding of the mechanism responsible for this form of experimental hepatic fibrosis.