This paper reviews the role of the acute phase response and of cytokines in clinical bone marrow transplantation. Data are discussed from the literature and from the authors experience which show that measurement of C-reactive protein is a rather non-specific marker of tissue injury, but that it is elevated in graft-versus-host disease, and especially in infection. Cytokines are clearly implicated in several aspects of transplantation, and tumour necrosis factor in particular may be important. Although there are some data which associate high TNF levels with severe graft-versus-host disease, this association may not hold true in individual patients.