Study of the chemical constituents of the stems of Garcinia schomburgkiana Pierre (Guttiferae), collected in Thailand, led to the isolation and identification of five known compounds and two new biphenyl derivatives, schomburgbiphenyl A (1) and B (2). Six phenolic compounds isolated from this plant were screened for their cell growth inhibition activity using several human leukemia cell lines. One compound, oblongifolin C (7), showed significant cytotoxic activity towards Jurkat, NALM6, K562 and HPB-ALL cells.