Objectives: Mutations in CALR (calreticulin) have been discovered in 50% to 80% of JAK2 (Janus kinase 2) and MPL (myeloproliferative leukemia protein) wild-type patients with Philadelphia-negative myeloproliferative neoplasm (MPNs). We evaluate the performance of a monoclonal antibody for immunohistochemical detection of CALR mutations. Methods: A computerized archival search was performed for cases of non-chronic myeloid leukemia (CML) MPNs with available CALR and JAK2 V617F mutational analysis data. Bone marrow biopsy specimens were stained with monoclonal antibody CAL2, and the percentage of stained megakwyocytes was calculated. In select cases, double immunofluorescence staining was done with CAL2 and each of the following: CD61, myeloperoxidase, CD34, and glycophorin A. Results: We studied 38 bone marrow biopsy specimens of non-CML MPNs (primal)) myelofibrosis, n = 21; essential thrombocythemia, n = 15; and n = 2 post polycythemia vera myelofibrosis) from 31 patients. All eight bone marrow biopsy specimens from patients with mutant CALR showed strong cytoplasmic staining of the megakaryocytes (83.5%; range, 50%-98%; median, 87%) with the CAL2 antibody. Double immunofluorescence staining of the small mononuclear cells seen in CALR mutant cases revealed them to be myeloid blasts. Conclusions: Immunohistochemistry in routinely processed bone marrow biopsy specimens for mutated CALR is feasible and accurately identifies mutated cases, including rare cases with additional driver mutations.