The antibody against calretinin was produced in mice by immunization with recombinant human calretinin-22k (1). Calretinin 22k is an alternative splice product of the calretinin gene and identical with calretinin up to Arg178. After fusion, hybridoma cells were screened with human recombinant calretinin as target and the clone 6B3 was selected. The antibody 6B3 recognizes an epitope within the first 4 EF-hands domains common to both calretinin and calretinin-22k (1). It was evaluated for specificity and potency: a) by Biotin-Avidin labeling of cryostate-, vibratome- and paraffin-sections of 4% paraformaldehyde (or 10% buffered formalin) fixed brains b) by immunoenzymatic labelling of immunoblots c) by immunohistochemistry on tissue of calretinin knock-out mice. The product is a monoclonal antibody against calretinin a calcium-binding protein of the EFhand family related to calbindin D-28k and calmodulin (2). The antibody reacts specifically with calretinin in tissue originating from human and rat. This antibody does not cross-react with calbindin D-28k or other known calcium binding-proteins, as determined by its distribution in the brain, as well as by immunoblots. The antibody recognizes calretinin in human mesotheliomas (3,4).