Colony-stimulating activity in murine mixed leukocyte cultures. 
Mixed leukocyte cultures (MLC) were prepared utilizing mouse spleen cells from strains with: (1) major histocompatibility complex (MHC) differences, (2) many non-MHC gene differences, and (3) only a few non-MHC gene differences.  After 4 days, the conditioned media (CM) of MLC from (1) and (2) above were found to contain high granulocyte/macrophage colony-stimulating activity (CSA) when assayed in methylcellulose clonal cultures.  Also the stimulation index (SI) of CSA was elevated on day 4 of incubation, even though 3H-Thymidine (3HTdR) incorporation and SI as measured by 3HTdR increased at 4 and 6 days, respectively.  However, MLC-CM from mouse strains with only a few non-MHC gene differences contained low titers of CSA after 4 days in culture, and this was paralleled by a low SI.  For MLC with MHC differences, and with many non-MHC gene differences, the response detected by an increase in CSA production appeared more promptly and was perhaps more sensitive when compared with 3HTdR incorporation in the mixed leukocyte reaction (MLR).  From these results, we conclude that the presence of CSA in MLC-CM at 4 days of culture represents a good marker for the degree of MLR, and we postulate that the CSA titer in MLC-CM may be relevant for the prediction of graft-versus-host reactions (GvHR).