The effect of the cleanliness of environmental air on the microbial contamination of a simulated i.v.-admixture during its preparation by aseptic transfer was studied under three conditions: (i) in a laminar air flow (LAF) bench situated in a class 1000 clean room, (ii) in an LAF bench in a microbiology laboratory and (iii) on a bench in an ordinary laboratory. Three thousand bottles were manually filled with 10 ml of a nutrient broth under each of the three conditions. The liquid was transferred by means of a syringe and a needle that pierced through the rubber closures of the bottles. The numbers of contaminated bottles under those three conditions were 1, 1 and 2, respectively. Background contamination had no demonstrable effect on the incidence of contamination, provided that preparation of the simulated i.v.-admixture solution was performed by skilled personnel in an LAF cabinet and air contact is avoided.