This study presents the sequential morphologic regeneration of graft-vs.-host (GVH)-induced dysplastic thymuses in long-term survivors of GVH reactions. GVH reactions were induced in adult C57BL/6xAF1 (B6AF1) hybrids by injecting 20 x 10 (6) A strain parental lymphoid cells (PLC). Starting on day 30 after GVH induction, five to ten animals were randomly selected from a pool of GVH-reactive mice and killed at various times. Each animal was tested for thymic histology and T cell functions. Thymuses taken on day 30 after GVH induction displayed severe dysplasia as characterized by lymphocytic depletion, complete effacement of cortico-medullary demarcation, and reduction and total loss of medullary epithelial cells or both. Starting by days 60-70 after GVH induction, at least four stages of thymic regeneration were identified. Day 60-70 thymuses displayed cortical regeneration and the reappearance of cortico-medullary demarcation. The medulla of these thymuses, although containing dark individual epithelial cells and numerous lymphocytes, was devoid of pale epithelial cells (stage 1). The medulla of thymuses on day 100 after GVH induction displayed a few sparcely distributed pale epithelial cells and numerous lymphocytes as well as dark epithelial cells (stage 2). The medulla of thymuses examined 130 days after GVH induction displayed numerous pale individual epithelial cells and a few pale epithelial cell clusters. Such thymuses also showed a reduction in the number of medullary lymphocytes (stage 3). Finally, the medulla of thymuses 150-160 days after GVH induction displayed numerous pale epithelial cell clusters and Hassall's bodies. These thymuses were indistinguishable from normal adult thymuses (stage 4). All of the animals tested up to day 130 after GVH induction showed no significant T cell function. Animals displaying stage 4 of thymic regeneration showed significant proliferative responses to T cell mitogen, concanavalin A (conA), and six of ten animals also displayed a few plaque forming cells (PFC) to sheep red blood cells (SRBC) in their spleens. Furthermore, all animals (10 of 10) killed on day 180 after GVH induction displayed significant T cell functions.(ABSTRACT TRUNCATED AT 400 WORDS)