Preliminary studies suggested that the use of compost tea made from spent mushroom substrate (SMS) may be regarded as a potential method for biologically controlling dry bubble disease in button mushroom. The aim of this study was to assess the effect of SMS compost tea on the host, the button mushroom, to ascertain whether the addition of these water extracts has a toxic effect on Agaricus bisporus mycelium growth and on mushroom yield. In vitro experiments showed that the addition of SMS compost tea to the culture medium inoculated with a mushroom spawn grain did not have an inhibitory effect on A. bisporus mycelial growth. The effect of compost teas on the quantitative production parameters of A. bisporus (yield, unitary weight, biological efficiency and earliness) was tested in a cropping trial, applying the compost teas to the casing in three different drench applications. Quantitative production parameters were not significantly affected by the compost tea treatments although there was a slight delay of 0.8-1.4 days in the harvest time of the first flush. These results suggest that compost teas have no fungitoxic effect on A. bisporus so that they can be considered a suitable biocontrol substance for the control of dry bubble disease.