The invention includes compositions of stem and progenitor cells recovered from bone marrow or cord blood containing most of the viable CD34+ cells and substantially depleted of red blood cells resident in the original sample, without any xenobiotic additives to aid cell separation. The invention also includes a system and method for preparing the compositions. The system includes a bag set and a processing device, which utilizes an optical sensor, microcontroller, servo motor, accelerometer, load cell, and battery. The system and method utilize centrifugation to stratify the cells into layers and then separate and transfer the stem cells into a stem cell bag. The processing device's microcontroller receives input from the device's accelerometer, load cell and optical sensor to direct the metering valve in the bag set to open and close to permit the transfer of as many stems cells as possible with as few red cells as possible.