Cellular immune recovery was studied in 67 patients who had survived for one month to more than 6 years after bone marrow transplantation (BMT). From a number of immunological parameters the mitogens phytohemagglutinin, concanavalin A and rabbit antibodies to beta 2 microglobulin (A-beta 2m) were chosen because of relevant increases in lymphocyte responses with time after BMT for analysis with exponential functions (covariance analysis). Patients with younger donors had a markedly higher lymphocyte response level and a faster rate of increase with time than patients with older donors. High recipient age was also significantly associated with a depressed level of immunological response but to a lesser degree than donor age. Patients with chronic graft-versus-host disease (GVHD) had a more severely impaired response compared to patients without chronic GVHD. Furthermore, patients with GVHD showed a negative development over time with the A-beta 2m test. Lymphocytes from patients receiving a low donor marrow cell dose showed impaired response levels and rate of recovery compared to the high-dose group although the differences were not significant. Patients treated with prednisolone for grade I GVHD had depressed lymphocyte stimulations for up to 1 1/2 years post BMT compared to patients without GVHD and not receiving prednisolone. Patients with non-viral infections seemed to have more depressed lymphocyte responses than non-infected patients. Patients undergoing cytomegalovirus infections had lower response levels for all three mitogens.