Donor leukocyte infusions (DLI) have been used effectively to induce remission in patients who relapse after BMT. Using CD34+ cell immunoaffinity enrichment, donor T cells may be captured in the unadsorbed (residual) fraction and we assessed this as a potential source of functional T cells for post-BMT immunotherapy. We extended our study to compare CD34+ cell selection and antibody-mediated cell lysis using Campath-1M and measured T cell-depletion, CD34+ cell recovery and relative progenitor proliferative potential. The recovery of CD3+ cells (responsive to IL-2 or PHA) in the unadsorbed fraction was 84+/-12% (mean+/-s.d.) using a laboratory scale CD34+ cell selection process (CEPRATE LC). The immunoselected (CD34+ cell enriched) product contained 55+/-12% of the starting CD34+ cells (purity, 75+/-6%) with recoveries of 44+/-12% and 42+/-13% for CFU-GM and BFU-E respectively. T cell depletion was 99.8+/-0.2% (FACS) and the frequency of clonable T cells estimated at 1:640 (limiting dilution assay). In comparison, Campath-1M-treated marrow samples gave recoveries of CD34+ cells, CFU-GM and BFU-E of 50+/-7%, 78+/-20% and 79+/-18%, respectively. The frequency of clonable T cells was 1:2700 despite an estimated T cell depletion of 98.4+/-1.9%. Data obtained from four BM harvests processed on the clinical grade CEPRATE SC system was comparable in every respect to the laboratory scale system. The yield of 1259 +/- 222 x 10(6) CD3+ cells in the unadsorbed fraction would allow for multiple graded incremental T cell aliquots for DLI for patients with acute leukaemia.