This paper provides the first published information on indoor concentrations of micro-organisms in a building in Rio de Janeiro, Brazil. The building in question was a 1,000 m2, twenty-nine-story building with a central air conditioning system. Concentrations and genera of bacteria and fungi were determined in several offices using a 6-stage impactor (Andersen sampler) during the summer season. Several gram-positive (Micrococcus, Bacillus, Staphylococcus, Streptococcus) and gram-negative (Pseudomonas, Serratia, Klebsiella) genus of bacteria were detected. Of the 13 fungi identified indoors, Cladosporium, Penicillium, Trichoderma and Aspergillus were predominant. Total bacteria and fungi counts showed a definite pattern in a comparison of results for the indoor and outdoor air. Therefore, the source of these micro-organisms is probably the outdoor air, from where they were transported indoors.