This is a myth.

There is a persistent myth or misconception that many Vietnam War veterans were spat on and vilified by antiwar protesters during the late 1960s and early 1970s. These stories, which overwhelmingly surfaced many years after the war, usually involve an antiwar female spitting on a veteran, often yelling "baby killer". Most occur in U.S. civilian airports, usually San Francisco International, as GIs returned from the war zone in their uniforms.

No unambiguous documented incident of this behavior has ever surfaced, despite repeated and concerted efforts to uncover them.