Background: The Mitochondrial Membrane Permeability Transition (MMPT) pore is an important target for the development of cytotoxic drugs because the release of cytochrome c and the irreversible opening of the pore is a point of no return for apoptosis to take place. A. boonei is a perennial plant that is quite ubiquitous in rain forest and sub-Saharan regions and it is used in folklore medicine as an anti-malarial decoction. Objective: To assess the effects of Vacuum Liquid Chromatography (VLC) chloroform (100%) fraction purified from the stem bark extract of A. boonei on MMPT pore of mitochondria isolated from livers of male rats in the presence and absence of added calcium ions. Materials and Methods: The in vitro effects of the VLC chloroform fraction on MMPT were assessed by monitoring the amplitude swelling spectrophotometrically as decreases in absorbance at 540 nm. Results: The MMPT pore was opened by six folds in the presence of calcium than in the absence of calcium. Spermine, a standard inhibitor of the pore almost completely reversed the effect of calcium thus indicating that the mitochondria were intact ab initio and suitable for use. Although calcium-induced pore opening was significantly inhibited by the VLC fraction in a concentration-dependent manner by 78.24, 88.55, 90.27 and 91.04% at 2.5, 5, 10 and 20 μg/ml, respectively, pre-incubation of mitochondria for 5, 10 and 15 minutes on ice and in the absence of calcium caused MMPT pore opening by two, five, and four folds, respectively, at 1.25 μg/ml and two, five and two folds, respectively at 2.5 μg/ml. Pre-incubation for these periods at 5 μg/ml caused the pore to open by two, seven and five folds, respectively, and four, eight and four folds at 10 μg/ml, respectively. Conclusion: These findings suggest that the bioactive agents in this fraction may find use in situations that require modulation of apoptosis.