Sera from halothane hepatitis patients have been shown to contain antibodies that react with several trifluoroacetylated proteins (100 kDa, 80 kDa, 63 kDa, 59 kDa, 57 kDa, and 54 kDa) purified from the livers of halothane treated rats. These findings suggest that similar trifluoroacetylated proteins are the immunogens responsible for the formation of the patients' antibodies and perhaps the subsequent development of hepatitis. This year a nearly full-length cDNA clone of the 63 kDa protein has been isolated from a Lambda gt11 rat liver cDNA library. Based upon the encoded sequence of this clone and the sequences of the N-terminal and several internal peptides of the purified protein, 63 kDa has been identified as calreticulin. Genomic DNA preparations from rat liver were probed with a fragment encompassing the cloned open reading frame. Strong homology was found to a single genomic region, with limited homology to a second region detected after prolonged autoradiography, indicating that the 63 kDa protein is not part of a multi-gene family. Immunoblotting of several tissues with the anti-63 kDa antibody indicated that the protein was present in all tissues of body, with highest concentrations being in the liver, testes, and adipose tissue. Calreticulin is a calcium binding protein of the endoplasmic reticulum in non-muscle cells and the sarcoplasmic reticulum in muscle cells. Calreticulin is thought to have a role in calcium homeostasis by acting as a storage reservoir of calcium. Since the present study has shown that calreticulin can be a target of the reactive metabolites of drugs, this raises the possibility that the calcium binding capacity of this protein can be altered by drugs and as a result play a role in the toxicity produced not only by halothane, but also by other drugs.