We have developed a novel subcutaneous sponge matrix model in major histocompatibility complex (MHC) homozygous SLAb/b inbred pigs to study lymphocyte-endothelial cell interactions during inflammation. Polyether sponges were implanted subcutaneously and left for 12 days before injection of proinflammatory agonists. Implanted sponges became highly vascularized and showed markedly increased uptake of i.v.-injected 51Cr-labelled lymphocytes 5 hr after injection of tumour necrosis factor-alpha (TNF-alpha) (3000 U) or phytohaemagglutinin (PHA) (37 micrograms). Lower levels of traffic were seen in sponges 5 hr after injection with interleukin-1 alpha (IL-1 alpha) (3000 U) and no significant traffic occurred in sponges injected with phorbol 12-myristate 13-acetate (PMA) (15 ng) at 5 hr or PHA at 24 hr (compared to sponges injected with medium alone). Electron microscopy of control sponges revealed low numbers of infiltrating leucocytes and relatively 'flat' endothelium. Many more infiltrating leucocytes were present in PHA-injected sponges. However, no ultrastructural evidence was seen of any significant difference between control and activated endothelium. Immunocytochemistry of frozen sections from sponges showed that E-selectin expression was up-regulated markedly by TNF-alpha and PHA at 5 hr, only moderately by IL-1 alpha at 5 hr, and not at all by PMA at 5 hr. By 24 hr in PHA-injected sponges E-selectin expression had fallen markedly from the level seen at 5 hr. Flow cytometric analysis of cellular infiltrates dispersed from sponges injected with TNF-alpha, PHA, IL-1 alpha or medium alone, revealed differences in lymphocyte subset populations. The infiltrate in sponges injected with TNF-alpha 5 hr before removal was dominated by high numbers of CD2+ lymphocytes, whereas the infiltrate induced by PHA showed relatively higher levels of CD2- CD4-CD8- gamma delta T-cell receptor+ (TCR+) T cells revealed by population-specific monoclonal antibodies (mAb). This model, which permits harvesting of leucocytes and endothelial cells for manipulation in vitro, will be useful for the study of leucocyte-endothelial cell interactions in subacute and chronic inflammation.