The antimycobacterial activity of twenty South African medicinal plants were investigated using two methods commonly used; the conventional agar plate method and the BACTEC radiometric method. Fourteen of the twenty acetone extracts of medicinal plants used to treat pulmonary diseases showed inhibitory activity at a concentration of 0.5 mg/ml against a sensitive strain of Mycobacterium tuberculosis using the conventional agar plate method. These fourteen extracts were also tested against M tuberculosis by the BACTEC radiometric method against a sensitive as well as a strain resistant to the drugs isoniazid and rifampin. Eight plants showed activity against both the strains at a concentration of 1.0 mg/ml. Susceptibility testing of M tuberculosis by the agar plate method is reliable, economical, and reproducible whereas the BACTEC radiometric method is much faster and probably more accurate than the agar plate method. A cytotoxicity assay of the fourteen plants on primary vervet monkey kidney cells showed that the crude acetone extracts of E. natalensis was the least cytotoxic extract with significant antimycobacterial properties. It was therefore, chosen for the isolation of active compound(s). An antibacterial assay of the water and acetone extracts of the roots of E. natalensis showed that they inhibited the growth of Gram-positive bacteria at concentrations ranging between 0.1 and 6.0 mg/ml. The water extract did not exert any inhibitory action on Gram-negative bacteria while the acetone extract showed inhibitory activity at a concentration of 5.0 mg/ml. The MIC of diospyrin, isolated from E. natalensis, was found to be 100 µg/ml for a drug-sensitive and a number of drug-resistant strains of M. tuberculosis and Gram-positive bacterial species. An antiviral investigation of the crude extracts of E. natalensis showed that the water extract of the roots of the plant inhibited the replication of herpes simplex virus type 1 moderately at a concentration of 0.2 mg/ml whereas, acetone extract at concentrations ranging from 0.1 to 0.02 mg/ml. Diospyrin exhibited no inhibitory effect against the virus. The MIC of 7-methyljuglone, isolated from E. natalensis, was found to be 50 µg/ml for both drug-sensitive and drug-resistant strains of M. tuberculosis. The compound inhibited the growth of Gram-positive bacterial species at concentrations ranging from 50 to 100 µg/ml. No inhibitory effect of the compound was observed on any Gram-negative bacteria at the highest concentration tested. A significant synergistic effect of the two naphthoquinones was observed against M tuberculosis and some of the bacterial species. MICs obtained were 10 µg/ml and 50 µg/ml for M tuberculosis and the bacterial species respectively. No synergistic effect was observed on any Gram-negative bacterial species investigated. In view of the encouraging results obtained from this study on the biological activity of the two naphthoquinones; diospyrin and 7 -methyljuglone, it appears that the compounds deserve further investigation in order to explore its potential as antimycobacterial agents.