Biological contamination of indoor air in office buildings can consist of bacteria and their spores, fungi, fungal spores and metabolic products from microorganisms. Microorganisms can enter buildings from the outside, but they can also come from materials that are already inside. The aim of this study was to assess and identify the airborne microorganisms in the indoor air of the office buildings during spring-summer and autumn winter seasons. The two office buildings selected for this study were located in the centre of Warsaw. Viable microbiological colonies were collected with a MAS 100 Eco sampler. The most common bacterial species were Gram-positive Microccocus spp., Staphylococcus epidermidis, and Bacillus spp rods. Several species of fungi were detected, such as Aspergillus spp., Penicillum spp. Data from this study have shown that the levels of bacteria and fungi in all office buildings were below the proposed threshold limit values. There were significant differences in microbiological air contaminants between naturally ventilated and air-conditioned office buildings. The total number of bacteria and fungi was higher in naturally ventilated than in air-conditioned office buildings.