The inhibitory activity of oolonghomobisflavan A was strongest among assamicains, oolonghomobisflavans, and thea-
sinensins. It is suggested that a methylene bridge linking the 8,8′-positions of two flavan units was responsible for the higher activities. It has been reported that oolonghomobisflavans were isolable from oolong tea but not from green tea or black tea; therefore, many active compounds, such as these, may only exist in oolong tea.