Operation Sea-Spray: The Secret Bacteria Test on San Francisco

In September 1950, the U.S. Navy sent a ship offshore from San Francisco and sprayed Serratia marcescens into the air. The military wanted to measure how a biological agent would spread through a coastal city during a possible attack from the sea or air. Residents received no warning. They did not consent and did not know they were part of a test.

The military chose Serratia because it was easy to culture and often produced a red colour that laboratories could identify in samples. At the time, the military considered it a relatively safe tracer. Still, hundreds of thousands of people inhaled an agent that had been released on purpose. The Navy used a real city instead of a laboratory chamber because an indoor test could not reproduce sea wind, fog, tall buildings, windows, vehicles, or the number of people living there.