A quantitative analysis of a recalled contaminated lot of heparin (HP) sodium injection United States Pharmacopeial (USP) was undertaken in response to the controversy regarding the exact nature of the contaminant involved in the HP crisis. A mass balance analysis of the formulated drug product was performed. After freeze-drying, a 1 ml vial for injection afforded 54.8 ± 0.3 mg of dry solids. The excipients, sodium chloride and residual benzyl alcohol, accounted for 11.4 ± 0.5 mg and 0.9 ± 0.5 mg, respectively. Active pharmaceutical ingredient (API) represented 41.5 ± 1.0 mg, corresponding to 75.7 wt% of dry mass. Exhaustive treatment of API with specific enzymes, heparin lyases and/or chondroitin lyases was used to close mass balance. HP represented 30.5 ± 0.5 mg, corresponding to 73.5 wt% of the API. Dermatan sulfate (DS) impurity represented 1.7 ± 0.3 mg, corresponding to 4.1 wt% of the API. Contaminant, 9.3 ± 0.1 mg corresponding to 22.4 wt% of API, was found in the contaminated formulated drug product. The recovery of contaminant was close to quantitative (95.6-100 wt%). A single contaminant was unambiguously identified as oversulfated chondroitin sulfate (OSCS).