Jackson played for two Class A teams in 1966, with the Broncs for just 12 games, and then 56 games with Modesto in the California League, where he hit 21 homers. He began 1967 with the Birmingham A's in the Double-A Southern League in Birmingham, Alabama, where Jackson got his first taste of racism, being one of only a few blacks on the team. He credits the team's manager at the time, John McNamara, for helping him through that difficult season.