Perhaps the greatest risk of catastrophe in developing a chemo-resistant cell line is from inadvertent contamination of one cell line with another cell line. Microbial contamination of cell cultures is rarely an issue because several replicate flasks of cells are cultured simultaneously and if one flask is contaminated there are other flasks at the same stage of development. At worst, cultures can be restarted using cells cryopreserved on a regular basis during cell line development. Therefore, ChemoScreen considers the most important element in development of a chemo-resistant cell line is the use of good cell culture practices. Practices include (1) dedicated cell culture medium for each cell line; (2) never placing flasks of different cultures of chemo-resistant cells in the biological safety cabinet at the same time; (3) using only tips and pipets with filters to prevent aerosol cross-contamination; (4) swabbing all pipettors with 70% propanol prior to use with each different cell line; (5) clearly labeled cell culture flasks and compound vials.