Leucocyte procoagulant activity (PCA) has previously been advocated as a useful measurement of cell-mediated immune reactivity. The assay is, however, susceptible to inter- and intra-experimental variation. This investigation identifies several factors which influence the stability and reproducibility of the test. Major factors which affect the recalcification time of plasma, include plasma lability, medium/buffer pH and both the nature and concentration of the indicator cells used in the assay. C. parvum-induced mouse peritoneal exudate cells have been used as a novel source of mononuclear phagocytes in the generation of PCA. Their sensitivity as indicator cells has been demonstrated by their responsiveness to stimulation by phytomitogen, endotoxin, and lymphokine (macrophage procoagulant-inducing factor). A simple test based on antigen-induced PCA of these cells, has provided an in vitro index of in vivo sensitisation to sheep red blood cells. Optimisation and standardisation of conditions detailed in this report for estimating PCA, renders the assay of value in monitoring lymphocyte and macrophage activation.