Thirteen samples of heparin, including one sample of non–clinical-grade heparin (control heparin 1), representing both suspect heparin lots and control lots, were analyzed in a blinded fashion for both the presence of OSCS and the ability to activate kallikrein. The presence of OSCS was detected and quantified as described elsewhere.6 The amidolytic activity of kallikrein was assessed at various concentrations of heparin, as indicated. Lots A through D and F through J contained OSCS. Samples from Lot E, as well as controls 1 through 3, contained no detectable OSCS. Lots A through G and Lot J were recalled on January 17, 2008. T bars indicate standard deviations of replicate measurements. ND denotes not detected.