The inv(16)(p13q22) or, less commonly the t(16;16)(p13;q22), is characteristic of acute myeloid leukemia (AML) with abnormal bone marrow eosinophils, also known as AML-M4Eo. This abnormality creates a fusion gene, 5' core binding factor beta (CBF-beta) gene and the 3' MYH11 gene, the latter encoding smooth muscle myosin heavy chain gene (SMMHC). Detection of this abnormality is important for diagnosis and is most commonly done by cytogenetics or molecular methods. In this study, we determined the utility of immunohistochemical and immunofluorescence methods using a rabbit polyclonal antibody (AH107) against the C-terminus of the CBFbeta-SMMHC chimeric protein for diagnosis of AML-M4Eo. Thirty-nine AML-M4Eo cases and 55 cases of other types of AML were evaluated. Immunohistochemical analysis of routinely processed paraffin-embedded bone marrow sections showed that CBFbeta-SMMHC staining is predominantly nuclear in all cases of AML-M4Eo and is not nuclear in other AML types. Four cases of AML-M4Eo double-stained for CBFbeta-SMMHC and CD34 showed the fusion protein in CD34-positive blasts. Indirect immunofluorescence analysis of fresh bone marrow aspirate smears showed that AML-M4Eo blasts have a distinct nuclear microgranular or fine-speckled pattern of staining, with or without faint cytoplasmic staining. By contrast, other types of AML and normal bone marrow specimens were either negative or had a nonspecific pattern of staining. In summary, immunostaining for CBFbeta-SMMHC using either immunohistochemical or immunofluorescense analysis as described here reveals a distinctive pattern of staining for AML-M4Eo. This approach is a specific, reliable, and convenient alternative to cytogenetic and molecular methods for the diagnosis of AML-M4Eo and may be particularly helpful in cases with indeterminate histologic features or in cases in which cytogenetic and molecular studies are either uninformative or not available.