"One day our society will come to respect the sanitation worker if it is to survive, for the person who picks up our garbage is in the final analysis as significant as the physician, for if he doesn't do his job, diseases are rampant. All labor has dignity."
Source: pp. 352-353 in The Autobiography of Martin Luther King, Jr. Edited by Clayborne Carson (1998). Grand Central Publishing.
Note: In March 1968, the month before he was assassinated, King went to Memphis to support 1000+ sanitation workers on strike.