Described is a new immunosuppressant, "Tsukubamycin A" (I), being the C-9 dihydro analog of FK-506, and a new process for preparing the immunosuppressant, "Tsukubamycin B" (II), being the C-21 propyl analog of FK-506, produced as by-products under fermentation conditions for producing FK-506 utilizing the microorganism, Streptomyces tsukubaensis No. 9993 (FERM BP-927). The macrolide immunosuppressants are useful in preventing human host rejection of foreign organ transplants, e.g. bone marrow and heart transplants.