The eutaneous basophil hypersensitivity (CBH) reaction in guinea pig is similar but with some differences to delayed hypersensitivity. Distinctive features of the CBH reaction are that numerous basophils accumulate at the reaction site. Basophilic leucocytes, therefore, may play an important role as a cellular component in CBH. Morphological features of the CBH reaction over time were observed by light and electron microscopy. The following results were obtained: 1) The number of circulating basophilic leucocytes increased during the CBH skin reaction. 2) The CBH skin reaction turned red 12hr after challenge of the antigen, and reached a maximum in size at 24hr. Then it disappeared completely at 96hr. At 20min. after rechallenge of the antigen, a crimson color was displayed at the site of the CBH skin reaction. 3) Basophils, mononuclear cells and neutrophils were infiltrated into the site of the CBH reaction and these cells reached a maximum number 24hr after challenge of the antigen. Mast cells and their granules recovered after 96hr. 4) Degranulation of infiltrating basophils, induced especially with rechallenge of the antigen, was observed as halo-formation and extracellular release of granules by light and electron microscopy. In conclusion, these findings indicate that basophils in CBH reaction are sensitized with homocytotropic antibody, and that mast cells may have a factor for accumulation of basophilic leucocytes at the skin reaction site.