OBJECTIVE We investigated the effects of two peptides from the ras-p21 protein, corresponding to residues 35-47 (PNC-7) and 96-110 (PNC-2), on two ras-transformed human cancer cell lines, HT1080 fibrosarcoma and MIAPaCa-2 pancreatic cancer cell lines. In prior studies, we found that both peptides block oncogenic, but not insulin-activated wild-type, ras-p21-induced oocyte maturation. When linked to a transporter penetratin peptide, these peptides induce reversion of ras-transformed rat pancreatic cancer cells (TUC-3) to the untransformed phenotype. METHODS These peptides and a control peptide, linked to a penetratin peptide, were incubated with each cell lines. Cell counts were obtained over several weeks. The cause of cell death was determined by measuring caspase as an indicator of apoptosis and lactate dehydrogenase (LDH) as marker of necrosis. Since both peptides block the phosphorylation of jun-N-terminal kinase (JNK) in oocytes, we blotted cell lysates of the two cancer cell lines for the levels of phosphorylated JNK to determine if the peptides reduced these levels. RESULTS We find that both peptides, but not control peptides linked to the penetratin sequence, induce phenotypic reversion of the HT-1080 cell line but cause tumor cell necrosis of the MIA-PaCa-2 cell line. On the other hand, neither peptide has any effect on the viability of an untransformed pancreatic acinar cell line, BMRPA1. We find that, while total JNK levels remain constant during peptide treatment, phosphorylated JNK levels decrease dramatically, consistent with the mechanisms of action of these peptides. CONCLUSIONS We conclude that these peptides block tumor but not normal cell growth likely by blocking oncogenic ras-p21-induced phosphorylation of JNK, an essential step on the oncogenic ras-p21-protein pathway. These peptides are therefore promising as possible anti-tumor agents.