Abstract: An improved composition and method for detecting fibrinogen, fibrinogen split products and/or fibrin split products in blood comprises utilizing killed, dyed Staphylococcus aureus cells which can be prepared by either of two methods. In the first method, Staphylococcus aureus organisms are incubated in a nutrient medium containing a polyalkylene glycol having a molecular weight of from 1000 to 5000 to which is added triphenyltetrazolium chloride; the growing organisms reduce the triphenyltetrazolium chloride to triphenylformazan which imparts coloration to the cells; the dyed Staphylococcus aureus cells are killed and substantially all untrapped dye is removed. In an alternate method, a suspension of Fast Black Salt K is added to a suspension of killed Staphylococcus aureus cells, and the dyed cells which result are washed to remove substantially all unfixed dye. The killed, dyed Staphylococcus aureus cells prepared by either method are suspended in an imidazole buffer to maintain a pH of 7.4.