Retention times and ultraviolet spectra were used to identify various adulterants and synthetic byproducts in 40 street samples seized by the police in Granada, Spain during 1988 and 1989. Of the street samples, 75 percent contained heroin and 25 percent cocaine. Only two samples contained both heroin and cocaine. Adulterants were detected in 67 percent of the samples containing heroin and in 60 percent of the samples containing cocaine. The most common adulterants in heroin samples were caffeine, procaine, phenobarbital, and acetaminophen, while benzocaine and lidocaine were the most common adulterants in cocaine samples. All heroin samples contained acetylmorphine; in 23 samples, however, the concentration was less than 10 percent. Morphine was present in about 25 percent of the samples, in concentrations of up to 4.3 percent. Good agreement was obtained between adulterants and impurities determined by HPLC/DAD, GC/NPD, and gas chromatography/mass spectrometry (GC/MS). It is concluded that the combined use of HPLC/DAD and GC/NPD as a routine analytical procedure for drug analysis may be a good alternative to GC/MS for forensic purposes. 11 references, 1 table, and 1 figure