Twenty-two methanol and chloroform crude extracts from Guttiferae (Hypericum hookerianum. Wight & Arn; Garcinia speciosa. Wall; Garcinia xanthochymus. Hook. F. ex T. Anderson; Cratoxylum formosum. subsp. pruniflorum. (Kurz) Gogel; and Calophyllum polyanthum. Wall ex Choisy) and Schisandraceae families (Schisandra verruculosa. Gagnap) collected from the northern region of Thailand were tested for antiproliferative activity on B₁₆F₁₀ (melanoma), HeLa (cervical carcinoma), and KB (epidermoid carcinoma) human cancer cell lines using the sulforhodamine-B (SRB) binding assay. All crude extracts showed an interesting antiproliferative activity with a dose-response relationship. The chloroform extract from leaves of G. speciosa. was the most potent in inhibiting cancer cell growth with the GI₅₀ of 4, 6.6, and 3.7 µg/mL in HeLa, KB, and B₁₆F₁₀ cell lines, which were 13-, 20- and 142-fold less potent than doxurubicin, respectively. The information from this study can explain the use of these plants in Thai traditional medicine and suggests the potential for further development of these extracts to new pharmaceuticals.