The basis for the clinical indications for the use of the plasma heparin assay for anticoagulation control are reviewed, and alternatives for currently used laboratory methods of monitoring heparin therapy in selected clinical situations are recommended. Many literature references support the role of plasma heparin assay with combined heparin and warfarin therapy, with combined heparin and recombinant tissue plasminogen activator therapy, in situations in which heparin resistance is noticed in the presence of a circulating anticoagulant, and in patients receiving heparin in an unrecognized manner, as well as with the use of the low molecular weight heparins. For now, however, in the absence of any prospective clinical trials establishing the superiority of the plasma heparin assay compared with the APTT, the recommendations outlined are presented as guidelines for the more specific monitoring of heparin therapy using currently available techniques.