Abstract: 1. Crystalline rhein and emodin are stable. In solution they are relatively heatstable, but very sensitive to light especially in the case of rhein in alkaline solution. After standing emodin was found to be very unstable in neutral solution. Rhein and emodin lose their antibacterial activity in the course of decomposition. 2. Rhein and emodin inhibit S. aureus at a concentration of 15 and 10γ/ml fluid medium respectively. Even at a concentration as high as 200γ/ml, only bacteriostatic but no bactericidal, effect is noted. 3. Serial passage of S. aureus in media containing rhein fails to make it drug resistant. 4. Riboflavin, folic acid, nicotinic acid, glutathione, xanthine, and guanosine exhibited a strong antagonistic action against both rhein and emodin.