Peptides have been designed to be T cell tolerogenic for the principal allergens of the cat. These have been administered in several dosage programs to cat-sensitive patients in multicenter blinded studies. In contrast to proteins in standard extracts, IgE sensitization to peptides is an uncommon event. Pretreatment prick tests with peptides will identify the occasional sensitized patient. Other side reactions consist of allergic symptoms occurring on the day of injections. These become less severe with subsequent injections and are easily treatable with antihistamines or bronchodilators, depending on the symptoms. Treatment with cat peptides ameliorated symptoms that occur upon exposure to cats 1-6 or more weeks later. A 2-week course of 4 injections is the most effective of the regimens so far tried. T-cell-active peptides offer a promising low-risk alternative for specific treatment of respiratory allergies.