To elucidate the molecular mechanisms involved in the delayed-type hypersensitivity (DTH) reaction, we performed a differential display analysis to identify genes whose expression was elevated in guinea pig DTH reaction-elicited skin-infiltrating cells. One of genes isolated was identified as a guinea pig I3 gene that encodes a polypeptide of 125 amino acids. The amino acid sequence was above 90% identical to that of human, mouse, and rat I3 protein. Although I3 was originally identified as a gene expressed in mouse brain, its mRNA was widely expressed in various guinea pig tissues and immune cells such as spleen cells and macrophages, but was hardly detected in thymic cells. I3 protein was also detected in immune cells, except for thymic cells. As expected from the amino acid sequence, part of the I3 protein was located on the cell surface. Furthermore, green fluorescent protein-tagged I3 protein showed a particulate localization in the cytoplasm and partly colocalized with lysosomes and late endosomes, coincident with the presence of typical endosomal/ lysosomal-targeting motifs. These results suggested that I3 may be involved in trafficking via the plasma membrane or in granule-related functions.