The modified mixed leukocyte reaction (MMLR) test consists of the standard MLR (SMLR) test to which interleukin-4 (IL-4) has been added. It is a sensitive procedure capable of detecting alloreactivity not detected by the SMLR. In the present study we applied the MMLR test to unrelated bone marrow transplantation (BMT) in an attempt to predict graft versus host disease (GVHD) and graft rejection (GR) by detecting alloreactivity between recipient/donor pairs otherwise found to be fully matched (HLA class I A and B tested by serology; class II DRB1 and DQB1 by sequence specific oligonucleotide probes [SSOP]) and by studying the relationship of MMLR alloreactivity and HLA-C disparity in the prediction of transplant related complications. Thirty-five patients transplanted from unrelated donors were included in the study. The MMLR test was seen to correlate with the incidence of transplant related complications, as of the 19 positive, cases 12 (63%) developed acute GVHD and 7 (37%) GR, while of the 16 negative cases only 5 (31%) developed GVHD (4 acute, 1 chronic) (p = 0.0001) and 2 (12.5%) GR. No such correlation was seen between the SMLR and the incidence of transplant related complications: the SMLR test was positive in only 4 (11%) cases (all of which developed GVHD or GR) but of the 31 negative cases 22 (71%) also developed GVHD or GR. Reactivity in the MMLR also correlated with molecular HLA-C disparity (p = 0.015): While of the 19 positive cases 10 (53%) had molecular HLA-C disparity, of the 16 cases with negative MMLR, 14 (87.5%) were matched for molecular HLA-C. Two-way analysis confirmed that patients with positive MMLR transplanted from HLA-C mismatched donors were more likely to develop post BMT complications, including GVHD and GR, than patients with negative MMLR transplanted from HLA-C matched donors (r = +0.70) (p = 0.001). We conclude that the MMLR test may be a useful tool in the prediction of transplant related complications such as GVHD and GR, post unrelated BMT. Moreover, the MMLR test, in conjunction with molecular HLA-C typing, may improve unrelated donor selection.