In order to obtain derivatives of rapamycin, a total of 28 bacterial and 72 fungal strains were screened for their ability to transform rapamycin. In the course of this screening, the already known derivatives 39-o-demethylrapamycin, 27-o-demethylrapamycin, 16-o-demethylrapamycin, and the 40-o-phosphoric ester of rapamycin were detected and isolated out of fermentations with Streptomyces rimosus ATCC 28893 or Thamnidium elegans ATCC 8997. Biotransformation of rapamycin using Syncephalastrum racemosus ATCC 1332B, Gliocladium deliquescens ATCC 10097, or Bacillus subtilis ATCC 55060 yielded the conversion products 24-hydroxyrapamycin, secorapamycin A, B, and C, and 16-o-demethylsecorapamycin B. None of these derivatives exhibited a stronger immunosuppressive effect than the parent compound; however, in the case of 24-hydroxyrapamycin and 40-o-phosphoric ester of rapamycin, a FKBP-binding affinity comparable to rapamycin was observed.