Spaceflight has adverse effects on the human body that pose health risks to astronauts spending extended time in space missions. For clinical monitoring of astronauts and for in-flight biomedical research, laboratory instruments must be available in the spaceflight environment. Currently, no instrument has been shown to be capable of generating a white blood cell (WBC) count and differential during spaceflight to our knowledge, although this is a medical requirement of the National Aeronautics and Space Administration (NASA). We evaluated a compact hematology analyzer for compatibility with a zero-gravity environment. We performed analyses in reduced-gravity during parabolic flight. Herein, we describe our engineering evaluation and report the reduced-gravity validation data we collected. The hematology analyzer we tested met the basic requirements for use in spaceflight and should be capable of accurately measuring WBC parameters aboard the International Space Station.