Basophils are the main cell component in cutaneous basophil hypersensitivity (CBH) reactions, but the role of basophils and the factors which gather them into CBH reaction sites are unknown. To investigate these problems, we induced CBH reactions in guinea pigs and observed basophils and mast cells in the skin reaction sites using light and electron microscopy. Basophils infiltrated into CBH reaction site appeared at 5 h after the challenge with antigen, increased till 48 h and decreased thereafter. On the other hand, the number of mast cells and their granules decreased after the challenge with antigen and reached a minimum at 48 h, but recovered at 96 h. The changes in the number of basophils and mast cells were complementary. This result suggested that the granules of mast cells may have a factor to gather basophils into the CBH skin reaction site. Furthermore, basophils infiltrated in the CBH reaction site were degranulated by the rechallenge with antigen, which was considered to be by an anaphylactic reaction.