Five commercial reagents for determining the activated partial thromboplastin time were compared on an instrument with photometric clot detection. The activating agents employed in these reagents are ellagic acid, celite, kaolin and a new agent, colloidal silica. The reagent containing colloidal silica was the most sensitive to low levels of heparin. It also was the only reagent with substantial buffering capacity, and in precision on repetitive tests it was equal to or better than any of the other reagents evaluated. Although there were no striking differences among the reagents in sensitivities to moderate deficiencies in individual coagulation factors, the colloidal silica reagent was most sensitive to factor V and the reagents containing ellagic acid were significantly less sensitive to factors X and XII.