The estimated number of employees in the United Stated screened annually for illicit drugs is approximately 20 million, with marijuana being the most frequently abused drug. Urine adulterants provide an opportunity for illicit drug users to obtain a false-negative result on commonly used primary drug screening methods such as the enzyme multiplied immunoassay technique and the fluorescence polarized immunoassay technique (FPIA). Typical chemical adulterants such as nitrites are easily detected or render the urine specimen invalid as defined in the proposed SAMHSA guidelines for specimen validity testing based on creatinine, specific gravity, and pH. Papain is a cysteine protease with intrinsic ester hydrolysis capability. The primary metabolite of the psychoactive chemical in marijuana, 11-norcarboxy-Delta9-tetrahydrocannibinol (THC-COOH), was assayed by FPIA in concentrations ranging from 25 to 500 ng/mL, at pH values ranging from 4.5 to 8, over the course of 3 days with papain concentrations ranging from 0 to 10 mg/mL. FPIA analysis of other frequently abused drugs: amphetamines, barbiturates, benzodiazepines, cocaine, opiates, and phencyclidine, along with gas chromatography-mass spectrometry (GC-MS) of THC-COOH and high-pressure liquid chromatography-ultraviolet detection (HPLC-UV) of nordiazepam was performed in order to determine if the mechanism of urine adulteration by papain was analyte specific. Control and adulterated urine specimens (n = 30) were assayed for creatinine, specific gravity, and pH to determine if papain rendered the specimens invalid based on the proposed SAMHSA guidelines. There was a direct pH, temperature, and time-dependent correlate between the increase in papain concentration and the decrease in THC-COOH concentration from the untreated control groups (p < 0.01). The average 72-h THC-COOH concentration decrease at pH 6.2 with a papain concentration of 10 mg/mL was 50%. Papain did not significantly decrease the concentration of the other drugs analyzed with the exception of nordiazepam. GC-MS of THC-COOH and HPLC-UV of nordiazepam revealed a 66% and 24% decrease in concentration of the respective analyte with 10 mg/mL papain after 24 h at room temperature (approximately 23 degrees C). No adulterated specimens were rendered invalid based on the SAMHSA guidelines. Immediate FPIA analysis is suggested to minimize the interfering effects of papain with regards to primary drug screening.