Mark Donohue typed letter signed on Roger Penske Enterprises Inc. letterhead, dated 14 March 1973. Letter is addressed to Jim McGregor, a fellow race car driver who wrote to Donahue for advice. Reads in part, ''...I can very easily understand your frustrations in the racing business and very quickly related to what you are going thru since in some ways, I went thru the same thing myself starting in the same way but you must be careful not to take the wrong attitude concerning racing...the only measure of success in racing and the only hope that you or anybody else has to become a successful racing driver is to start winning races up to the highest level. No one is ever going to become a professional driver if he doesn't win races...'' Letter on two pages is signed ''Mark'' in blue ink on page two. Measures 8.5'' x 11'' with two minor horizontal folds, else near fine. Includes original envelope.