As part of an International Cooperative Biodiversity Group (ICBG) program and a continuing search for antiproliferative natural products from the Madagascar rainforest, and a collaborative research project established between Virginia Tech and the Institute for Hepatitis and Virus Research (IHVR) focusing on searching for bioactive natural products from tropical forests in South Africa, 20 extracts were selected for investigation based on their antiproliferative activities against A2780 human ovarian cancer cell line or antimalarial activities against the Dd2 strain of Plasmodium falciparum. Bioassay-guided fractionation of seven of the extracts yielded twenty new compounds and twenty-four known compounds, and their structures were elucidated by using a combination of 1D (1H and 13C) and 2D NMR spectroscopy including COSY, HASQC, HMQC, HMBC, and NOESY sequences, mass spectrometry, UV, IR, ECD, optical rotation, and chemical conversions. In addition, ten known compounds were isolated from another five of the extracts, while studies on the remaining extracts were suspended due to loss of activity, unworkable small amounts of material, or low structural interest. The plants and their metabolites are discussed in the following order: five new antimalarial 5,6-dihydro-𝛼-pyrones and six bicyclic tetrahydro-𝛼-pyrone derivatives from Cryptocarya rigidifolia (Lauraceae); two new and five known antiproliferative lignans from Cleistanthus boivinianus (Phyllanthaceae); two new and two known antiproliferative sesquiterpenes lactones from Piptocoma antillana (Asteraceae); one new antiproliferative 1,4-naphthoquinone, one known antiproliferative isoflovonoid, and five known antiproliferative stilbenoids from Stuhlmannia moavi (Leguminosae); four known antiproliferative bisbenzylisoquinoline alkaloids from Anisocycla grandidieri (Menispermaceae); one new and two known antiproliferative butanolides, and two new antiproliferative secobutanolides from Ocotea macrocarpa (Lauraceae); one new antiproliferative and five known antiproliferative diterpenoids from Malleastrum rakotozafyi (Meliceae); and 10 known compounds from Monoporus sp. (Myrsinaceae), Premna corymbosa (Verbenaceae), Premna perplexanes (Verbenaceae), Epallage longipes (Asteraceae), and Cinnamosma fragrans (Canellaceae). / Ph. D.