The therapeutical effectiveness of a calf thymic derivative (thymomodulin), orally administrable, in patients suffering from perennial allergic rhinitis is described. The use of thymomodulin for 4 months at the dosage of 120 mg daily in 20 patients significantly reduced (p less than 0.001), if compared with the previous year, the monthly mean frequency of allergic episodes. Also objective symptoms improved (p less than 0.05). The clinical course was associated with the normalization of IgE plasmatic levels (p less than 0.001) and consequently the mean of RAST and PRICK tests scores decreased (p less than 0.001). Eosinophil count was significantly reduced (p less than 0.001), while IgG serum levels were increased (p less than 0.02).