After the failure of his salt business in Saginaw, Flagler returned to Bellevue in 1866 and reentered the grain business as a commission merchant with the Harkness Grain Company. During this time he worked to pay back his stepbrother Stephen Harkness. Through this business, Flagler became acquainted with John D. Rockefeller, who worked as a commission agent with Hewitt and Tuttle for the Harkness Grain Company. By the mid-1860s, Cleveland had become the center of the oil refining industry in America and Rockefeller left the grain business to start his own oil refinery. Rockefeller worked in association with chemist and inventor Samuel Andrews.