Introduction: Adhesion molecules play a central role in the immune defence. The aim of the present study was to define the reactivity pattern of anti-adhesion molecule monoclonal antibodies (MAb) with sub-populations of porcine mononuclear cells (PBMC).Materials and Methods: Expression of adhesion molecules on PBMC was followed with time (0-96 hr.) in culture (unstimulated), or after stimulation with mitogens ConA, LPS or PWM. Adhesion molecule expression, and the leukocyte sub-populations were characterized by two- and three colour flow cytometry (FCM) using cross-reactive anti-human adhesion molecule monoclonal antibodies (MAb) or porcine specific MAb.Results: Resting monocytic cells expressed a high level of β2 integrins and CD44. CD49d and CD49e expression on these cells was dim to low with CD49d, showing a skew towards lower expression. As monocytes differentiated into macrophages upon in vitro culture, there was a down-regulation of both β1 integrins. CD29 was also expressed as for a single population, and was independent of the differentiation stage of the myeloid cells. LPS stimulation enhanced the down-regulation of β1 integrin expression seen during monocyte-to-macrophage differentiation.All porcine lymphocytes expressed a high level of β2 integrins and CD44. The majority of lymphocytes labelled with the anti-CD49d MAb; a minority of freshly isolated cells could also be stained with the anti-CD49e MAb. This pattern was repeated when CD4 + and γ/δ T cell sub-populations were analysed. CD8 + lymphocytes had a skew towards higher CD49d expression. These sub-populations of T-lymphocytes were all CD49e d i m / l o w to CD49e - . On resting B lymphocytes, a low expression of CD49e was seen. Following ConA or PWM stimulation, T and B lymphocytes respectively were clearly seen to up-regulate β2 integrin and CD44 expression. An initial down-regulation of CD49d followed by a late up-regulation was noted. The CD4 + cells formed an exception to this pattern displaying a more rapid increase of CD49d expression after mitogen stimulation. With CD49e, the CD4 + , CD8 + and γ/δ T cells all showed an up-regulation, as did PWM stimulated B cells. For the CD4 + sub-population this was manifest by a late (after 72 hr.) increase. For CD8 + and γ/δ T cells the up-regulation was observed from 24 hr. onwards.Conclusion: These results demonstrate how β1-, β2 integrin and CD44 expression on porcine mononuclear cells differ between subpopulations and upon mitogen stimulation. They relate in some but not all characteristics to that which has been noted with human mononuclear cells.