The growth-inhibiting effects of Pinus densiflpora leaf-derived materials on nine human intestinal bacteria were investigated using the impregnated paper disk method, and their activities were compared with those of 13 commercially available terpenes. The biologically active constituent of the extract of P densiflora leaf was characterized as the monoterpene (1R)-(+)-$alpha$-pinene by various spectroscopic analyses. Responses varied according to bacterial strain, chemicals, and dose. At 10 mg/disk, limonene and (1R)-(+)-$alpha$-pinene strongly inhibited the growth of Clostridium perfringens, Staphylococcus aureus, and Escherichia coli, without adverse effects on the growth of five lactic acid-bacteria (Bifidobacterium adolescentis, B. bifidum, B. longum, Lactobacillus acidophilus, and L. casei). Little or no inhibition against seven bacteria was observed with anethole, borneol, camphor, caryophyllene, 1,8-cineole, estragole, linalool, and $alpha$-terpineol. Structure-activity relationship revealed that (1R)-(+)-$alpha$-pinene had more growth-inhibiting activity against C. perfringens than (1R)-(+)-$eta$-, (1S-(-)-$alpha$-, and (1S-(-)-$eta$-pinenes. Furthermore, the growth-inhibition against L. casei was much more pronounced in (1R)-(+)-$eta$- and (In-(-)-$eta$-pinenes than (1R)-(+)-$alpha$- and (1S)-(-)-$alpha$-pinenes. These results indicate that the (+)-$alpha$ form seems to be required against C. perfringens and $eta$ form against L. casei for growth-inhibiting activity. Morphologically, most strains of C. perfringens were damaged and disappeared at 5 and 2 mg/disk of (1R)-(+)-$alpha$-pinene. Morphological study revealed that (1R)-(+)-$alpha$-pinene had more growth-inhibiting activity against C. perfringens than (1R)-(+)-$eta$-, (1S)-(-)-$alpha$-, and (1S)-(-)-$eta$-pinenes. As naturally occurring growth-inhibiting agents, the Pinus leaf-derived materials described above could be useful preventive agents against diseases caused by harmful intestinal bacteria such as clostridia.