A total of 30 random samples of commercial mayonnaise were collected from different retailers in Assiut city. To assess their quality, the samples were examined microbiologically for the incidence and counts of aerobic plate count (APC), psychrotrophs, thermoduric, enterococci, coliforms, fecal coliforms, Escherichia coli, Bacillus cereus, Staphylococcus aureus, anaerobes and yeasts & molds. The obtained results verify that the total bacterial, psychrotrophs, thermoduric and enterococci counts averaged 3x104, 1.7x10', 2.7x104 and 2.9x109/g of the examined mayonnaise samples, respectively. B. cereus could be isolated from 20%, in numbers averaged 1x10°/g, of the examined mayonnaise samples. Yeasts & molds contaminated 26.67% of the examined mayonnaise samples and existed in numbers averaged 2.2x10-/g of the samples. All the examined mayonnaise samples failed to yield coliforms (less than 3/g), and therefore fecal coliforms and E. coli could not be recovered from all of the examined samples. Also, S. aureus and anaerobes could not be detected in any of mayonnaise samples examined. The results prove that the examined commercial mayonnaise samples sold in Assiut city are of quite good quality and considered as microbiologically safe products. Although, the microbial loads are below the hazard point, the health hazard of such microorganisms still exists if they are allowed to grow and multiply, and that what was studied through the second part of the present study, in which the survival and viability of Salmonella typhimurium, Escherichia coli O157:H7, Listeria monocytogenes, Staphylococcus aureus and Bacillus cereus in commercial mayonnaise kept at room temperature were studied. Each mayonnaise sample was inoculated separately with one of the mentioned pathogenic microorganisms and then incubated at room temperature (about 25°C) as commercial mayonnaise is usually distributed, shelved and stored at this temperature. After that, the inoculated mayonnaise was sampled after 6, 24, 48 and 72 hours. The results revealed the lethal effect of mayonnaise on the inoculated microorganisms although the survival and viability of L. monocytogenes and B. cereus was relatively longer than others. Suggestive hygienic measures for improving the quality of mayonnaise and also to safeguard the consumer were discussed.