Ten house dust extracts were examined for enzymatic activity using the Api-Zym System. Comparisons were also made with extracts of mites and molds. The results show a high degree of heterogeneity between dust extracts from different companies and also to a lesser extent between lots of dust extracts from the same company, indicating variability of source material for extraction. It seems possible to detect enzymatic activity in house dust extracts associated with molds but more difficult to do the same with mites. The correlation between enzymatic activity and allergenic activity of house dust cannot be made from these data.