SummaryAn extract of dust from the outlet filters of a mouse isolator was used as a basis for determining the source of inhalant allergens for subjects sensitive to this species. The antigenic components, identified by crossed immunoelectrophoresis (XIE), were compared to those found in extracts of other mouse‐derived source materials, i.e. urine, fur, dander and saliva. Of the eight dust components, one (Ag 1) was identified as antigenically identical to the major urinary pre‐albumin whilst the others were detected in fur, and to a lesser extent dander and saliva. None of the dust antigens was detected as a component of food or bedding.Crossed radio‐immunoelectrophoresis (XRIE), performed using sera from a group of fifteen mouse‐allergic subjects (positive by RAST to mouse extracts), identified seven of the dust antigens as IgE‐binding components. Antigens 1 and 3 were reactive with all the sera tested and have, therefore, been termed the‘major’allergens. Varied responses were obtained to the other‘minor’antigens.Ag 1 (urinary pre‐albumin) and Ag 3 were detected in all samples of mouse dust studied. RAST and RAST inhibition also indicated the presence of urinary prealbumin. These findings suggest that the major mouse inhalant allergens may be derived predominantly from urine and secretions originating in the skin and