The chloroform-soluble fraction of the leaves of R. ferrugineum showed cytotoxic activity for the HL-60 cell line at a concentration of 10 μg/mL after 24 h, decreasing cell viability to ∼1% (data not shown). Since the toxicity of Rhododendron species has been attributed to the presence of grayanane diterpenes, the contribution of grayanotoxins to the cytotoxicity of the chloroform-soluble fraction of R. ferrugineum was investigated. TLC