A poorly differentiated transitional cell carcinoma in C3H/He mice results from the oral ingestion of the urinary tract carcinogen FANFT. This model, designated MBT2, is readily transplantable into syngeneic animals and has proven to be very useful in the development of chemotherapy. Prior to the use of this model for the testing of potential immunotherapeutic strategies, we have attempted to characterize the immunobiology of this tumor line. We report that the primary growth of this tumor in the footpad and its metastasis to lung are correlated with the development of increased numbers of suppressor cells, characterized by the expression of a surface histamine H2 receptor. These cells are originally evident in spleen and become maximal approximately 4 weeks after tumor implantation. This is followed by the migration of these cells from spleen to peripheral blood, an event that parallels the growth and eventual metastasis of this implanted transitional cell carcinoma. These events may have important significance for the development of immunomodulating therapy against bladder cancer.