Tumor cells are known to activate monocytes/macrophages and it has been shown that this stimulation was conferred by tumour-cell membranes. In order to analyse the relevant structures for tumor cell-specific TNF-induction monocytes from healthy donors were cultured in the presence of plasma membrane preparations from Jurkat or K562 cells. Both tumour cell lines revealed a monocyte-stimulating plasma membrane component of about 45 kDa. The TNF-inducing factor exhibited characteristics of a glycoprotein with the carbohydrate moiety as the structure responsible for stimulation. CD2, a glycosylated T-cell specific membrane component, was identified as being involved in monocyte activation in the case of the Jurkat cells whereas the identity of the activating structure on K562 cells is still unknown. From the data presented here indicating the importance of carbohydrate structures for monocyte activation we conclude that altered glycosylation of cell surface molecules of tumour cells might be responsible for tumour cell-induced monocyte stimulation.