Background Helminthiasis is a major limitation to the livestock industry in Africa. Haemonchus contortus is the singular most important helminth responsible for major economic losses in small ruminants. The high cost of anthelmintics to small farmers, resistance to available anthelmintics and residue problems in meat and milk consumed by humans further complicates matters. The use of plants and plant extracts as a possible source of new anthelmintics has received more interest in the last decade. Our aim was not to confirm the traditional use, but rather to determine activity of extracts. Based on our past experience acetone was used as extractant. Because it is cheaper and more reproducible to evaluate the activity of plant extracts, than doing animal studies, the activity of acetone leaf extracts of thirteen plant species used traditionally in ethnoveterinary medicine in South Africa were determined using the egg hatch assay and the larval development test. Cytotoxicity of these extracts was also evaluated using the MTT cellular assay. Results Extracts of three plant species i.e. Heteromorpha trifoliata, Maesa lanceolata and Leucosidea sericea had EC50 values of 0.62 mg/ml, 0.72 mg/ml and 1.08 mg/ml respectively for the egg hatch assay. Clausena anisata; (1.08 mg/ml) and Clerodendrum glabrum; (1.48 mg/ml) extracts were also active. In the larval development assay the H. trifoliata extract was the most effective with an EC50 of 0.64 mg/ml followed by L. sericea (1.27 mg/ml). The activities in the larval development test were generally lower in most plant species than the egg hatch assay. Based on the cytotoxicity results C. anisata was the least toxic with an LC50 of 0.17 mg/ml, while Cyathea dregei was the most toxic plant with an LC50 of 0.003 mg/ml. The C. anisata extract had the best selectivity index with a value of 0.10 and 0.08 for the two assays, followed by H. trifoliata and L. sericea with values of 0.07, 0.07 and 0.05, 0.04. The C. dregei extract had the worst selectivity index with a value of 0.00019 for both assays. Conclusion The result of this study indicates which species should be further investigated in depth for isolation of compounds.