Abstract Many species of plants in African countries are widely used in the rural communities where there is little or no access to modern medicine. However, the safety and effectiveness of these medicinal plants are poorly evaluated. The stem bark of Parkia biglobosa Jacq. and leaves of Ageratum conyzoides Linn. were investigated for their antibacterial and cytotoxic activities. The plant materials were extracted with 95% ethanol, and fractionated with petroleum ether, chloroform and ethyl acetate. The antibacterial effects of the extracts and fractions of the plant materials were assayed on the bacterial cultures of Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus, Methicillin Resistant Staphylococcus aureus (MRSA) and Clostridium perfringes. Ethanol extracts of P. biglobosa and A. conyzoides were screened for cytotoxicity using the 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyltetrazolium bromide (MTT) assay. Two cancer cell lines (SK-MES 1 and SK-LU 1) and one normal cell line (human skin fibroblast cell line, FS5) were used for the screening of the extracts and the fractions obtained. The ethanolic extracts and fractions of P. biglobosa and A. conyzoides showed the best activity against E. coli, S. aureus and MRSA. All fractions of A. conyzoides leaves have no activity against P. aeruginosa. Human lung cancer cell lines (SK-LU 1 and SK-MES 1) and human skin fibroblast cell line (FS5 cells) were treated with various concentrations (3.9μg/ml–2mg/ml) of the extracts and fractions for 24h. SK-MES 1 cells are more susceptible to treatment with the plant fractions. All the fractions of A. conyzoides leaves and the petroleum ether fraction of P. biglobosa were cytotoxic to SK-MES 1 cells, which to some extent may support their traditional inclusion in herbal preparations for treatment of cancer. The overall results provided evidence that the studied plant extracts might be potential sources of new antibacterial and anticancer drug.