Perilla frutescens BRITTON (perilla, Labiatae) is a medicinal herb prescribed in Saiboku-to, which is a Kampo formula effective for allergic diseases such as bronchial asthma. The present study was conducted to evaluate the anti-allergic effect of orally administered perilla decoction and to identify the active constituents using mice ear-passive cutaneous anaphylaxis (PCA)-reaction, which is one of the animal models for type I allergy. Perilla decoction significantly suppressed PCA-reaction, and the inhibition % at the dose of 500 mg/kg was 43%. The perilla decoction contains 5.3% of luteolin 7-O-[β-glucuronosyl(2→1)β-glucuronide], 1.6% of apigenin 7-O-[β-glucuronosyl(2→1)β-glucuronide], 0.49% of scutellarin, and 2.5% of rosmarinic acid (weight of compound/dried weight of perilla decoction %), respectively. When these constituents were orally administered to mice at the dose equivalent to 500 mg/kg of perilla decoction, rosmarinic acid and apigenin 7-O-[β-glucuronosyl(2→1)β-glucuronide] significantly suppressed PCA-reaction, and their inhibition % was 41% (p<0.01) and 32% (p<0.05), respectively. Since the inhibition % or perilla decoction and rosmarinic acid were nearly equal, the anti-allergic effect of perilla decoction depends primarily on rosmarinic acid. The standard Saiboku-to decoction contained 0.013% of rosmarinic acid, which was too low to exhibit anti-allergic activity in a daily dose of Saiboku-to in adults, suggesting that perilla would be prescribed in Saiboku-to to exhibit other pharmacological effects than its anti-allergic activity, such as a sedative.