A man believed to be the US' oldest worker has retired at the age of 104.

Science professor Ray Crist has stepped down from his teaching position at Messiah College in Grantham, Pennsylvania.

He began teaching there in 1970 after reaching Dickinson University's mandatory retirement age of 70.

In 1926 he received his doctorate in chemistry from Columbia University, and in the early 1940s worked on the project that separated isotopes of uranium for use in an atomic bomb.

He was a director of the project in 1945 and counted Albert Einstein among his personal friends, says the Pittsburgh Post-Gazette.

Professor Crist took just a token salary of $1-a-year from Messiah College.