beta 2 glycoprotein-I (beta 2GPI), a cofactor for antiphospholipid antibody (aPA) binding, binds to many anionic macromolecules including heparin. The nature of this interaction with heparin is not well understood and its effect on the purported biological functions of beta 2GPI is unknown. We have examined the interactions of dermatan sulphate (DS) and different pharmaceutical preparations of heparin with beta 2GPI by crossed immunoelectrophoresis (CIE) and investigated the effect of these agents on plasma levels of beta 2GPI antigen (beta 2GPI:Ag) by a standardised enzyme linked immunosorbent assay (ELISA). beta 2GPI aPA cofactor activity (beta 2GPI:Cof) was also measured using a modified solid phase anti-phosphatidylserine (aPS) ELISA. CIE results confirmed a heparin-beta 2GPI interaction with unfractionated (UF) heparin. beta 2GPI:Ag levels were unaffected by any of the preparations investigated. There were no significant differences in beta 2GPI:Cof activities of the samples containing LMW heparins or DS but levels of beta 2GPI:Cof were increased in samples containing UF sodium and calcium heparin preparations (0.5 IU/ml Monoparin, p < 0.05, and 10 IU/ml Liquemin and Calciparine, p < 0.05).