Glutaminyl cyclase (QC) is responsible for the presence of pyroglutamyl residues in many neuroendocrine peptides. An examination of the bovine tissue distribution of QC immunoreactivity, enzyme activity, and mRNA confirmed that QC was abundant in brain and pituitary by all three measures. However, enzymatic activity was considerably more widespread than either immunoreactivity or mRNA, suggesting multiple enzyme forms. Partially purified QC from bovine spleen differed significantly from the known bovine pituitary QC in physical and catalytic properties. We propose that this form of glutaminyl cyclase plays a role in the posttranslational processing of constitutively secreted pGlu-containing proteins.