The biotransformations of cinobufagin (1), an animal-originated bufadienolide, by cell suspension cultures of Catharanthus roseus and Platycodon grandiflorum were investigated. Incubation for 6 days of 1 with C. roseus yielded four products, desacetylcinobufotalin (2), 3-epi-desacetylcinobufagin (3), 1β-hydroxyl desacetylcinobufagin (4) and 3-epi-desacetylcinobufotalin (5), among which 4 is a new compound. Time course investigation revealed that the biotransformation rates of two major products, 2 and 3, reached their highest levels of 44.7 and 16.5%, respectively, on the third day after substrate administration. Compounds 2-5 showed more potent cytotoxic activities against HL-60 cell lines than the parent compound 1. A plausible biotransformation pathway is proposed to account for the formation of the observed products. From the culture supernatant of P. grandiflorum, 2 was isolated in 37.8% yield after 8 days of incubation with 1. Cinobufotalin (6) and desacetylcinobufagin (7) were also obtained as minor products. The two plant suspension cultures exhibited similar transformation patterns on compound 1.