Organic extracts of 24 selected plant species, used by Palestinian traditional healers to treat different illnesses and diseases, were tested for their anti-inflammatory and anti-tumoral activities. The plant selection was based on existing ethnobotanic information and interviews with local healers. The extracts of the plants under investigation were tested for their potential anti-tumor (cytotoxic) effect on the murine fibrosarcoma L929sA cells, and on the human breast cancer cells MDA-MB231 and MCF7. Cytotoxicity screening models provide important preliminary data to select plant extracts with potential antineoplastic properties. MTT (Tetrazolium blue) colorimetric assay was used to evaluate the reduction of viability of cell cultures in the presence or absence of the extracts. The extract from Withania somnifera, L. Dunal (Solanaceae) presented an IC(50) value at 24h of 150 and 60 microg/ml, on L929sA and MCF7 cells, respectively, while the extract from Psidium guajava L. (Myrtaceae) presented an IC(50) value at 24h of 55 microg/ml on MCF7 cells. Other extracts examined, like Laurus nobilis L. (Lauraceae) and Salvia fruticosa M. (Labiatae), also displayed a remarkable activity. Additionally, as the nuclear transcription factor NFkappaB regulates the expression of various genes that play critical roles in apoptosis and immunomodulation, we further investigated the effect of nine promising plant extracts, withheld from the first cell viability screening on NFkappaB activation. The extracts showed variable degrees of NFkappaB-inhibitory activity. Whereas Withania somnifera extract demonstrated the strongest NFkappaB-inhibitory activity, other extracts derived from Laurus nobilis, Psidium guajava and Foeniculum vulgare M. (Umbiliferrae) also revealed immunomodulatory NFkappaB activities. These species are good candidates for further activity-monitored fractionation to identify active constituents.