Despite the use of conventional chemoprophylaxis regimens, patients receiving unrelated-donor BMT are at high risk of developing severe acute GVHD. We evaluated a prophylactic regimen combining CsA, MTX and anti-CD5-ricin A chain immunotoxin (H65-RTA) in 31 patients; pentoxifylline was also given to reduce the anticipated nephrotoxicity of CsA. In most cases, planned doses of CsA, MTX and H65-RTA were given (i.e. to 77%, 77% and 93% of patients, respectively). Although fluid retention requiring diuretic therapy was frequent, only 1 patient had a > 10% unexplained increase in body weight during the first 21 days post-BMT. Also, while significant increase of the baseline serum creatinine was noted in 7 patients, none required dialysis. One patient suffered a reversible allergic reaction to the immunotoxin; no other side effects attributable to this regimen were observed. All but 2 patients engrafted (1 died of fungemia on d + 19 and the other had persistent leukemia) and no late graft failures were observed. Seventeen patients developed acute GVHD grade > or = II (probability, 58% [95% CI 41-76%]); 7 had grade > or = III (probability, 24% [95% CI 12-43%]). In the 27 patients who achieved stable engraftment and have survived beyond d + 100, the 3-year probability of developing chronic GVHD was 66% (95% CI 48-84%). As of the last follow-up prior to 01 May 1994, 13 patients are alive in CR and one in relapse; 9 of these patients are off all immunosuppressives and well. Four other patients relapsed and died, and 13 died of other transplant-related causes.(ABSTRACT TRUNCATED AT 250 WORDS)