Background: Serum alkaline phosphatase (AP) changes in men with metastatic prostate cancer have been described since the Nobel Prize winning discovery of the hormonal dependence of PC. High serum AP is independently associated with mortality in men with castration-resistant prostate cancer (CRPC) and reductions in AP with treatment are favorably prognostic. Transient rises in AP may occur during treatment initiation and reflect bone remodeling/healing. The source of serum AP in men with metastatic prostate cancer, however, remains unclear. We hypothesize that cancer cell osteomimicry and epithelial plasticity may contribute to serum AP elevations in these men. Methods: We examined the expression of AP in the circulating tumor cells (CTCs) of men with metastice CRPC on abiraterone acetate who had elevated serum AP to determine if one possible source of serum AP may be tumor-derived rather than host/bone derived. CTCs were isolated using the CellSearch method and EpCAM ferromagnetic capture, and only CD45 negative cells were considered CTCs. AP was evaluated using immunofluorescence and real time positive and negative controls. Liver function was evaluated to ensure lack of confounding with liver-derived AP. Results: We identified AP to be commonly expressed in the CTCs of men with CRPC who had high serum AP, with no expression in patient leukocytes. One patient had a transient rise in serum AP with no change in other liver enzymes during a dramatic response to abiraterone acetate while his prostate-specific antigen and CTC declined. AP expression was evident and common in his CTCs during this serum AP flare. A second patient was found to have common CTC AP expression in both CK+ and CK- CTCs. This patient had evidence of AR amplification, PTEN loss, and TMPRSS2-ERG fusion in his CK- and CK+ CTCs, accompanied by a high number of OB-cadherin + CTCs, indicating likely osteomimicry and plasticity of these clonal epithelial tumor cells. Conclusions: We provide clinical evidence that PC CTCs express AP, supporting the hypothesis that AP expression is associated with osteomimicry as part of the biology of epithelial plasticity in prostate cancer. These findings support the existence of multiple CTC phenotypes, the clinical significance of which is unknown.