Vaccine adjuvants stimulate the innate immune system and determine the outcome of the immune response induced. A better understanding of their action is therefore crucial to the development of new and safer vaccines. Monophosphoryl lipid A (MPL), a 'detoxified' version of lipolysaccharide, is a promising new adjuvant component in human vaccines. The present study uses an ovine lymphatic cannulation model to study cell recruitment and antigen transport from the injection site into the afferent lymph, and how this is modulated by co-injection with MPL. Compared with saline, MPL injections caused only minor variations in lymph flow and no difference in cell number migrating into the lymph. MPL did, however, cause a significantly increased recruitment of neutrophils and monocytes, but not dendritic cells (DC) into the lymph for the first 12 h. Soluble ovalbumin (OVA) antigen flowed freely into the lymph over a 24-h period and was slightly reduced at 6-9 h in the MPL-injected sites. OVA-coated fluorescent 1-μ beads were initially transported predominantly by neutrophils and, from 24 to 72 h, by DC. MPL induced an increased and more sustained transport of beads by neutrophils and monocytes although it did not increase the phagocytic capacity of these cells. In contrast to aluminium adjuvant, MPL did not increase bead transport by DC at the later time point. These studies provide important new insights in the in vivo action of different adjuvants and the initial events that set up an immune response after vaccination.