Early exposure to allergens is important for sensitization to inhalant allergens and it has been reported that there is a causal relationship between allergen exposure and bronchial asthma. We investigated the levels of major mite (Der p I/Der f I), cat (Fel d I) and dog (Can f I) allergen levels in dust collected from various locations in seven day-care centres (22 sections). The allergen levels were related to the characteristics of the day-care centres. Children and staff were questioned about contacts with animals, and cleaning personnel were asked about methods and frequency of cleaning. Mite allergen was found in nine of the 22 sections. The concentrations varied between < 16 ng/g and 106 ng/g dust (median < 16 ng/g). Mite allergen was not detected in any floor dust sample. Cat and dog allergen was detected in all centres and sections. The concentrations of dog allergen (median 4.3 micrograms/g; range < 60 ng-21 micrograms/g) were significantly higher (P < 0.05) than that of cat allergen (median 1.6 micrograms/g; range < 16 ng-22.8 micrograms/g). Higher amounts of both Fel d I and Can f I were observed on mattresses/sofas/cushion like toys and curtains than on tables/chairs and floors. The levels of cat or dog allergen on floors significantly correlated with the total number of children and staff with either a cat or a dog at home and or frequent contacts with them. Neither cleaning methods nor the frequency of cleaning influenced the allergen concentrations. The concentration of Fel d I was significantly lower (P < 0.05) in washed than in never washed curtains.(ABSTRACT TRUNCATED AT 250 WORDS)