The contamination rates associated with the preparation of medium-risk i.v. admixtures in a traditional practice site and in a class 1000 cleanroom were compared. Simulated product media fills served as the samples. Each investigator, a pharmacist and a pharmacy technician, prepared 500 vials and 500 small-volume parenteral (SVP) bags in five separate runs at a traditional practice site and in a cleanroom. United States Pharmacopeia chapter 797 medium-risk compounding procedures were followed, and strict adherence to aseptic technique was employed. Single-strength tryptic soy broth was substituted for the drug and diluent in the admixtures. Positive and negative controls were also prepared and stored for the duration of the study. The pharmacist and technician prepared a total of 4057 samples: 2027 samples (1014 vials and 1013 SVP bags) were prepared in a class 1000 cleanroom, and 2030 (1014 vials and 1016 SVP bags) were prepared at a traditional practice site. Contamination rates did not significantly differ between the traditional practice site (0.296%) and the cleanroom environment (0.344%) (p = 1.0). A significant difference in the number of contaminated samples was found between the two investigators (2 of 2057 were contaminated by the pharmacist and 11 of 2000 were contaminated by the technician) (p = 0.012). Contamination rates by the pharmacist (p = 1.0) and technician (p = 1.0) did not significantly differ between sites. The most important variable affecting microbial contamination of admixtures was the aseptic technique of personnel, not the environment in which the drugs were compounded.