In vitro priming of T cell with horse antilymphocyte globulin(HALG) results in cytokine release, and this has been associated with its clinical efficacy in patients with severe aplastic anaemia (SAA). Rabbit antithymocyte globulin (RATG) has been studied less extensively. In this study we compare the in vitro priming effect of HALG and RATG on purified normal marrow T cells: end-points of the study were 1) levels of TNF-alpha (TNF-α), IFN-gamma (IFN-γ GM-CSF in T cell supernatants, and 2) effect of T cell supernatants on colony formation with or without exogenous GM-CSF. TNF-α, IFN-γ and GM-CSF levels were comparable for HALG, RATG and phytohaemagglutinin (PHA). T cell supernatants showed comparable enhancement of colony formation in the presence of recombinant human GM-CSF (rhGM-CSF) and supported colony forming unit granulomacrophage (CFU-GM) growth in the absence of growth factor. This study shows that horse and rabbit derived ALG/ATG and PHA have a comparablein vitro priming effect on T cells: both agents should probably be tested for their clinical efficacy in SAA patients.