During in vitro incubation for several hours human circulating blood leucocytes developed procoagulant activity (PCA) which was identified as tissue thromboplastin-like activity and which remained specifically associated with monocytes. Adherence of monocytes to glass stimulated these cells to generate PCA. When cuprophane was used, both the degree of adherence of monocytes and PCA generation by these cells was significantly less. In addition, when the monocytes were incubated upon a monolayer of cultured endothelial cells only minor PCA generation was detectable. After incubation on glass the non-adherent monocytes had negligible PCA in comparison to the adherent monocytes. Protein synthesis inhibitors (cycloheximide and actinomycin D) inhibited PCA generation but did not affect monocyte adherence. These results demonstrate that adherence of monocytes is a stimulus for the generation of PCA.