Cocaine sold as a “street drug” in white powder usually contains several adulterants and diluents. The purpose of this research was to quantify cocaine content and to check the presence of adulterants in illicit samples seized by the Narcotic and Toxic Substances Regional Department of Alfenas and Varginha districts, Brazil, in the year 2001. The identification of cocaine and adulterants in 209 samples, was screened by thin-layer chromatography (TLC) and confirmed/quantified by gas chromatography/flame ionization detector (GC/FID). The pharmacologically inactive adulterants (starch, carbonates/bicarbonates and sugars) were analysed by qualitative tests or TLC. 88.9% of the samples analysed gave positive results for cocaine in concentrations between 4.3 and 87.1% of the powder. Active compounds detected were caffeine (50.2% of the samples, in concentrations ranging from 2.8 to 63.3% of the powder), lidocaine (65% of the samples, concentrations ranging 0.5–92%), and prilocaine (11% of the samples, concentrations ranging 1.4–20.7%). Carbonate/bicarbonate showed positive results in 41.2%, starch in 51.2% and sugars in 9.6% of the samples. The presence of other substances in cocaine street samples is worrying because they could modify or intensify signs and symptoms of the intoxication, acute or fatal, due to drug.