The present research study deals with the evaluation of sixty four methanolic and aqueous extracts of thirty Yemeni plants used in traditional medicine for their in vitro antiproliferative activity against three human cancer cell lines in a microtiter plate assay based on cellular staining with crystal violet, for their antimicrobial activity against antibiotic susceptible three Gram-positive, three Gram-negative bacterial and one fungal stains and three multiresistant Staphylococcus strains by the agar diffusion method and the determination of MIC against three Gram-positive bacteria with the broth micro-dilution assay, as well as for their antioxidant activity using the DPPH radical scavenging method. Furthermore the chemical composition of the methanolic extracts was determined by using chromatographic methods. As a result of this work, 12 Yemeni herbs namely Centaurothamus maximus, Costus arabicus, Cupressus sempervirens, Dichrocephala integrifolia, Euphorbia schimperi, Gomphocarpus fruticosus, Kanahia laniflora, Meriandera benghalensis, Pulicaria inuloides, Solanum glabratum, Tarconanthus camphoratus and Vernonia leopoldii demonstrated a noteworthy growth inhibitory effect against all cancer cell lines with IC50 values <50 microg/ml. Pronounced antimicrobial activity was observed only against Gram-positive bacteria among them multiresistant bacteria with inhibition zones >15 mm and MIC values <500 microg/ml, by 9 plants especially Centaurothamus maximus, Cupressus sempervirens, Enicostemma verticillare, Meriandera benghalensis, Nepeta deflersiana, Pulicaria inuloides, Tarconanthus camphoratus, Teucrium yemense and Vernonia leopoldii. Moreover, the methanolic extracts of Cupressus sempervirens, Meriandera benghalensis, Pulicaria inuloides and Rhus retinorrhaea showed a remarkable radical scavenging effect at low concentrations.