Working with laboratory animals (LA) may expose personnel to a complex mixtures containing, besides the well known allergens, microorganisms and their components that can lead to adverse health effects. This paper reports the results of a microbiological environmental monitoring carried out in a conventional laboratory facility housing about 7,000 animals including mice and rats. The primary objective was to measure and characterize airborne fungi, bacteria, endotoxins, and (1/3)-b-D-glucans through air sampling with the aim to determine which factors or working tasks were associated with the greatest level of exposure to biological agents. Bioaerosols were collected from seven animal rooms, a storage area, two washing rooms and some offices. In animal rooms air samples were taken over three consecutive days: the day in which the cages are changed, the day before and the day after changing cages. Mesophilic bacteria levels ranged from 2.5 to 885 CFU/m3, with the detection, during the change of cages, of Gram-negative bacteria belonging to risk group 2 (Escherichia coli, Enterobacter cloacae), while moulds and yeasts levels ranged from 0 to 335 CFU/m3. Mean concentrations of endotoxins and (1/3)-b-D-glucans were below the exposure limits in all workplaces except the storage area, where values respectively of 210.7 CFU/m3 and 5,145 pg/m3 were recorded during the preparation of litter and distribution of feed. The one-way ANOVA test showed a statistically significant increase (p-value <0.05) of all microbiological parameters during the change of cages. Our results identified the working phases carried out in animal rooms and in the storage area as the more "critical" with regard to the potential exposure to biological agents. The data suggest that rodent bedding could be a source of accumulation and release of endotoxins and fungal components. Further investigation is needed to estimate the personal exposure in order to implement proper control and prevention measures.