A microsomal butyrylesterase (L-I) was purified from the livers of male W rats treated with phenobarbital, and an antiserum against this purified L-I was raised in a rabbit. By the Ouchteriony double-diffusion test, a precipitin line was observed between the anti-L-I antiserum and each Triton X-100 extract of livers during development, regeneration after partial hepatectomy, and carcinogenesis and of hyperplastic nodules and hepatomas, all of which revealed L-I in their esterase isoenzyme patterns. These precipitin lines exhibited esterase activity. The fusion of the lines of these tissue extracts and that of the purified L-I indicated the presence of an antigen site common to their esterases. The extracts of adult and fetal livers and also of hepatomas resembling fetal liver in the esterase isoenzyme pattern did not produce a precipitin line with anti-L-I antiserum.