The one-way mixed lymphocyte reaction (MLR) is a useful model of the graft-vs.-host (GvH) response that occurs following bone-marrow transplantation (BMT). Previous studies of the MLR have shown high levels of type-1 cytokine production, such as IL-1, IL-6, IFN-gamma and TNF-alpha, but low or undetectable levels of type-2 cytokines, such as IL-4 and IL-10. Here, through establishing optimal conditions for the examination of levels and kinetics of a more definitive panel of type-1/type-2 cytokines (IL-4, IL-5, IL-10 and IL-13, IFN-gamma, TNF-alpha and the soluble IL-4 receptor) we show that, contrary to previously published data, the human alloresponse is truly heterogeneous, resulting in abundant type-2 as well as type-1 cytokine secretion. The kinetics of cytokine levels in the MLR show surprising complexity, suggesting a well-defined regulation as the alloresponse develops over time. Furthermore, each MLR responder:stimulator combination tested produces a composite cytokine profile that is intrinsic to that particular pairing. These combination-specific cytokine responses are reproducible when tested on multiple occasions over time. These data reveal a potential clinical application for the cytokine MLR in selecting donors for BMT with the least inflammatory cytokine profile. Additional analysis of this system reveals that the bulk of cytokine measured is both allospecific and T-cell-derived, with comparatively low levels produced through an autologous mechanism. Interestingly, although most of the cytokine detected is produced by CD45RO+ 'mature/activated' T cells, CD45RA+ 'naive' T cells are responsible for transient early production of IL-4. This novel finding suggests that naive T cells themselves could regulate type-1/type-2 developmental fate through an autocrine IL-4 mechanism.