When blood stored in glass vials sealed with natural rubber septums was screened for amphetamine, a contaminant was seen. The contamination originated from the rubber septums and was identified by GC/MS as N-ethylbenzenamine. Like amphetamine, N-ethylbenzenamine was extracted from strongly alkaline solutions. N-Ethylbenzenamine and amphetamine chromatographed with the same retention time on a packed Apiezon/KOH column and on nonpolar capillary columns. Separation was achieved on a medium polar capillary column. Trifluoroacetyl derivatives of amphetamine and N-ethylbenzenamine had the same retention time on a packed SP-2250 column, but separated on nonpolar capillary columns. The amounts of N-ethylbenzenamine found were high enough to give rise to significant false positive results if N-ethylbenzenamine was erroneously read as amphetamine. N-Ethylbenzenamine is believed to be formed by thermal decomposition of the vulcanizing agent zinc ethylphenyldithiocarbamate used in rubber production.