This antiserum was produced against recombinant rat calbindin D-28k (CB). It crossreacts with calbindin D-28k from many other species, including human, monkey, rat, mouse chicken and fish (Fig. 1). In immunoblots it recognizes a single band of approximately 27 -28 kDa. In samples with very high (10-fold higher than CB) concentrations of the related protein calretinin, a larger band (29 - 30 kDa) may appear in Western blots. Thus, depending on the dilution of the antiserum and/or sensitivity of the immunostaining, this other calcium-binding protein could be also revealed. In case of doubt, we suggest to switch to Swant’s monoclonal antibody 300 directed against calbindin D-28k, which does not recognize calretinin.