Three new alkaloids, Hirsutellones A, B, and C, are produced in cultures of fungus such as Hirsutella nivea strain BCC 2594 and Trichoderma sp. strain BCC 7579. The Hirsutellones exhibited potent growth inhibitory activity against Mycobacterium tuberculosis (H37Ra strain) with an MIC value of 0.78 .mu.g/ml, while showing weak or no cytotoxicity to mammalian cells. Therefore, Hirsutellones and pharmaceutical compositions containing Hirsutellones may be useful for the treatment of tuberculosis. Also described herein are methods of isolating the Hirsutellones.