Eight commercial cat dander extracts and two pelt extracts derived from mongrel and Siamese cats were compared. Cat allergn 1 and cat albumin were measured by radial immunodiffusion. Allergenic activity was evaluated by prick test and a modified radioallergosorbent test. In the latter, the dilution of each extract that produced 50% inhibition of binding of IgE antibodies to insolubilized cat allergen 1 (RAST 1) and insolubilized cat serum (RAST 2) was determind. The total non-dialysable solid content of the extracts did not correlate with any other parameter. Cat allergen 1 content determined by radial immunodiffusion correlated with average prick test results in ten cat-sensitive subjects and with RAST 1 activity. Cat albumin content correlated weakly with RAST 2 activity but not with any other measure of allergenic activity. Absorption of each extract with the gamma-globulin fraction of rabbit antiserum to cat allergen 1 significantly reduced prick test reactivity and RAST 1 activity, but not RAST 2 activity. These results indicate that cat allergen 1 is an important allergen in cat dander extracts and its measurement may be used to standardize the allergenic activity of such extracts.