We previously reported that the butanol (BuOH) fraction of the head of Panax ginseng exhibited gastroprotective activity in peptic and chronic ulcer models. In order to identify the active constituent, an activity-guided isolation of the BuOH faction was conducted with a HCI$.$ethanol-induced gastric lesion model. The BuOH fraction was passed through a silica-gel column using a chloroform-methanol gradient solvent system, and six fractions (frs. 1-6) were obtained. The active fr. 5 was further separated by silica-gel column, to yield 6 subfractions (subfrs. a-f). Subfr. d was composed of ginsenosides Re, Rc and $Rb_1$. The most active constituent was ginsenoside $Rb_1$ ($GRb_1$), a protopanaxadiol glycoside, which was investigated for its anti-ulcer effect. Gastric injury induced by HCI$.$ethanol, indomethacin and pyloric ligation (Shay ulcer) was apparently reduced with oral $GRb_1$ doses of 150 and 300 mg/kg. $GRb_1$ at these dosage significantly increased the amount of mucus secretion in an ethanol-induced model. The anti-ulcer effects were consistent with the result of histological examination. These results suggest that the major active constituent in the head of Panax ginseng is $GRb_1$ and that anti-ulcer effect is produced through an increase in mucus secretion.