Objective To compare results of immunohistochemical (IHC) assays for estrogen receptor (ER), progesterone receptor (PR) and human epidermal growth factor receptor 2 (HER2) performed on thrombin, formalin and Cellient cell blocks to those performed on tissue. Study Design Formalin, thrombin and Cellient cell blocks were prepared from cytologic samples obtained from resection specimens of 31 patients with invasive breast carcinoma. ER, PR, HER2 and MIB-1 (Ki-67) IHC stains were performed on all three types of cell blocks and compared to the same stains performed on the patient's paraffin-embedded biopsy or resection. Cell and tissue blocks with equivocal staining for HER2 were submitted for fluorescence in situ hybridization (FISH). Results Adequate Cellient blocks were obtained for all 31 cases. Comparison of results of ER IHC assays on all three types of cell blocks showed 100% correlation with tissue. Both Cellient and thrombin blocks showed 100% correlation with tissue for HER2 IHC and FISH results. The only statistically significant difference between cell block methods was found in PR staining, where false-negative results occurred with Cellient and thrombin blocks. Conclusion Breast biomarker IHC assays performed on Cellient blocks are reliable and correlate with tissue block results, particularly for ER and HER2, the most clinically important markers.