Micro-erythrosomes (mERs) are microscale containers (3 to 5 µm in diameter) derived from red blood cells (RBCs, also called erythrocytes). They are prepared by removing hemoglobin from RBCs and resuspending the empty structures in buffer. In this work, we focus on adding new functionalities to mERs, with both therapeutics and diagnostics in mind. In our main study, we demonstrate the use of mERs as “Killer Cells” to attack cancer. mERs are loaded with the enzyme glucose oxidase (GOx) and then incubated in vitro with a strain of head and neck cancer cells (15B). In the presence of glucose from external media, the Killer Cells generate hydrogen peroxide (H2O2). H2O2 is a reactive oxygen species (ROS) which induces the cancer cells to undergo apoptosis (programmed cell death). We find a reduction in 15B cell viability of over 80%. In ancillary studies, we explore strategies for the long-term retention of solutes in mERS. Specifically, the cationic biopolymer chitosan is adsorbed to the surfaces of mERs, and the anionic biopolymer alginate is encapsulated in their cores. Both strategies are able to extend the diffusion time for loaded solutes. Additionally, we have attempted to adapt mERs for use as MRI contrast agents by incorporating lipids containing gadolinium into the membrane. These studies lay the foundation for many mER applications and demonstrate their versatility.