Transgenic (Tg) mice, which carry human CD98 heavy chain (hCD98HC) gene, were made for the first time. Biochemical analyses of total protein, albumin, cholesterol, glutamic-oxaloacetic transaminase (GOT), glutamic-pyruvic transaminase (GPT) and alkaline phosphatase (ALP) in the sera were carried out. The expression of hCD98HC and murine CD98HC (mCD98HC) mRNA, and the tissue distribution of hCD98HC and mCD98HC were determined.The values of total protein, albumin, cholesterol, GOT, GPT and ALP of Tg mice were about the same as those of normal mice. The mCD98HC mRNA was highly expressed in all the tissues of normal mice examined except for skeletal muscle, where it was expressed rather slightly. hCD98HC mRNA of Tg mice was detected on cDNA prepared using oligo-dT primer, and the mRNA was clearly found in brain, kidney, skin, spleen, stomach, testis and thymus. However the mRNA was rather slight in liver, skeletal muscle and intestine, and almost nothing in heart. Immunofluorescence study of murine tissue sections using monoclonal antibody (mAb) directed against mCD98HC revealed that mCD98HC was clearly expressed in renal proximal tubules of kidney, follicular epitherium and keratinocyte of skin, and interstitial space including Leydig cell of testis. However, hCD98HC in Tg mice, determined by immunostaining using hCD98HC mAb, was not detected in any of the tissues examined.