Phytochemical studies on Peltodon longipes afforded the isolation of twelve known abietane diterpenes being the active principle. The main diterpene, 7α-acetoxyroyleanone exhibited a similar strong cytotoxic activity against a pancreatic and a melanoma cancer cell line as tanshinone IIa the main compound from Salvia miltiorrhiza. Structure–activity relationship study was undertaken including 21 structurally different abietane diterpenes. Phytochemical investigations of the n-hexane extract from the roots of Peltodon longipes (Lamiaceae) resulted in the isolation of 12 known abietane diterpenes (1–12). Structures were established on the basis of one and two dimensional nuclear magnetic resonance spectroscopic data (1H and 13C, COSY, HSQC and HMBC), electron ionization mass spectrometric analysis (EIMS) as well as comparison with data from literature. These compounds, as well as eight known diterpenes (13–19) from Salvia miltiorrhiza, and two from Salvia sahendica (20 and 21) were evaluated for their cytotoxic effects in human pancreatic (MIAPaCa-2) and melanoma (MV-3) tumor cell lines using the MTT assay. Tanshinone IIa (13), 7α-acetoxyroyleanone (1), 1,2-dihydrotanshinone (16) and cryptotanshinone (14) had the highest cytotoxic effects in MIAPaCa-2, displaying IC50 of 1.9, 4.7, 5.6, and 5.8μM, respectively. Structure–activity relationships of abietane diterpenoid quinones are discussed.