Samples of materials from mice habitat (air, feed, drinking water, litter) were investigated in order to estimate their bacterial content and number. The concentrations of bacteria in tested materials ranged from 6.1 × 10¹ to 3.1 × 10⁵ cfu/appropriate unit. Species of Micrococcus, Staphylococcus, Bacillus and Pseudomonas genera dominated. The cell populations of Gram-positive bacteria overdominated Gram-negative ones. Extended in time investigations confirm the long-term stability of microflora composition of tested materials. Stable presence of bacterial components in animals habitat influences on mice sera antigen specifity composition. Concentrations of specific antibacterial antibodies in tested sera were higher in elder mice and their prevalence were dependent on antigen availability.