The purpose of the present study was to identify which of the several possible defects in cell-surface-molecule interactions are responsible for diminished mouse helper T cell responses to xenoantigens. We measured primary mouse anti-monkey, anti-pig, and anti-human proliferation in vitro in experimental systems in which potential defects were partially corrected by lymphokine supplementation and/or the use of transgenic or hybridoma cell populations. We found that the diminished mouse helper T cell responses to xenoantigens result from at least two defects in cell-surface-molecule interactions between T cells and xenogeneic APCs, specifically TCR and/or CD8 interactions with xenogeneic class I MHC molecules and accessory molecule interactions with their ligands (probably LFA-1 with ICAM-1/ICAM-2 and/or LFA-2 with LFA-3). Other investigators have identified additional defects, such as in lymphokine function across species differences. Thus, there appear to be multiple defects responsible for the diminished cellular immune response to xenoantigens.