We have prepared antibodies against porcine liver betaine-homocysteine methyltransferase (BHMT; EC 2.1.1.5) and recently cloned cDNAs encoding the porcine and human liver enzymes. Porcine tissues were evaluated for BHMT expression by measuring catalytic activity and Western analysis. Liver and kidney were the only organs tested that had immunodetectable levels of BHMT, and these organs expressed high levels of enzyme activity. BHMT was expressed in the kidney cortex and not the medulla. Porcine pancreas, brain, heart, lung, and spleen were devoid of BHMT activity and immunodetectable protein. Human tissues were tested for BHMT expression by Northern analysis. Human liver and kidney were the only organs tested that expressed BHMT mRNA. Human pancreas, brain, heart, skeletal muscle, spleen, and placenta were devoid of BHMT mRNA. The human BHMT gene has been mapped to chromosome 5q13.1-q15.