An in vitro microbial challenge test has been developed to predict the likelihood of consumer contamination of baby shampoo. Four preservatives were tested in our study: the parbens Medcide D, Medcide PB, Sepicide HB. and isothiazolinone Methylisothiazolinone/Chloromethylisothiazolinone [MI/MCI]. These preservatives were tested separately and in combination. The challenge test involved inoculating the product with Micrococcus luteus, Staphylococcus aureus, Escherichia coli, Salmonella enterica, Pseudomonas aeruginosa, Aspergillus brasiliensis and Candida albicans. Inhibition growth of these microorganisms at each preservative concentration was followed over a 28 d period. The test was used to classify products as poorly preserved, marginally preserved, or well-preserved. Interestingly, it was the combination (0.1% Isothiazolinone [MI/MCI] and 0.1% Sepicide HB) which inhibited most the microbial growth of microorganims while preserving the physicochemical properties of the product. As a result, the challenge test described can be accurately used to predict the risk of consumer contamination of cosmetic products.