Disclosed is a method of combating Pneumocystis carinii Pneumonia in a subject in need of such treatment. The method comprises administering to the subject an effective Pneumocystis carinii-combating amount of a bis-benzimidazole compound such as bis[5-(2-imidazolyl)-2-benzimidazolyl]methane, 1,4-bis[5-2-imidazolyl)-2-benzimidazolyl]butane, or a pharmaceutically acceptable salt thereof. Pharmaceutical formulations for carrying out the method and novel compounds are also disclosed, along with methods of combating Giardia lamblia.