Recent evidence suggests a role for nanobacteria in a growing number of human diseases, including renal stone formation, cardiovascular diseases, and cancer. This large body of research studies promotes the view that nanobacteria are not only alive but that they are associated with disease pathogenesis. However, it is still unclear whether they represent novel life forms, overlooked nanometer-size bacteria, or some other primitive self-replicating microorganisms. Here, we report that CaCO3 precipitates prepared in vitro are remarkably similar to purported nanobacteria in terms of their uniformly sized, membrane-delineated vesicular shapes, with cellular division-like formations and aggregations in the form of colonies. The gradual appearance of nanobacteria-like particles in incubated human serum as well as the changes seen with their size and shape can be influenced and explained by introducing varying levels of CO2 and NaHCO3 as well as other conditions known to influence the precipitation of CaCO3. Western blotting reveals that the monoclonal antibodies, claimed to be specific for nanobacteria, react in fact with serum albumin. Furthermore, nanobacteria-like particles obtained from human blood are able to withstand high doses of γ-irradiation up to 30 kGy, and no bacterial DNA is found by performing broad-range PCR amplifications. Collectively, our results provide a more plausible abiotic explanation for the unusual properties of purported nanobacteria.