Recombinant human growth hormone (rhGH) improves growth in children after renal transplantation, but may be associated with augmented immune responses. To understand the effect of rhGH in transplantation, we evaluated the role of rhGH in a mixed leukocyte culture (MLC) in vitro. We demonstrated that PBMC isolated from normal adult volunteers cultured in an MLC in the presence of rhGH develop an augmented proliferative (25-400%) and cytotoxic (50-600%) response. Using in situ hybridization (ISH), we demonstrated that the frequency of cells expressing mRNA for IFNγ increased in the presence of rhGH (100-800%). In vitro responses of PBMC from adults in an MLC may only loosely reflect responses in vivo during pediatric transplantation. After transplantation, adults develop decreased responses to donor-specific antigens in an MLC (donor-specific hyporesponsiveness - DSH). We evaluated the donor-specific responses of 20 pediatric patients who had each received a renal allograft from one parent. Pediatric patients developed DSH similarly to adults; however, no correlation was seen between the amount of DSH and graft function. We also evaluated the expression/production of IFNγ and IL4 in response to donor-specific alloantigens. Patients exhibit marked DSH of IFNγ expression and production. However, IL4 production was seen in 8 out of 10 patients with normal renal function, but only 1 out of 7 patients with biopsy proven chronic rejection. Finally, we evaluated the effect of rhGH in vitro on DSH. Only 3 out of 20 patients developed augmented donor-specific responses in the presence of rhGH in vitro. rhGH augments proliferation, cytotoxicity and IFNγ expression during an MLC. After renal transplantation, rhGH augments donor-specific responses during an MLC in some pediatric patients. © Munksgaard, 1997.