Which Chicago blues bassist played on Fleetwood Mac in Chicago?

Willie Dixon was the Chicago blues bassist who played on Fleetwood Mac in Chicago.

Dixon performed upright bass during the sessions that produced the album, recorded at Chess Records' Ter-Mar Studios in Chicago on 4 January 1969. The project brought the young British blues band together with veteran Chicago musicians whose records had strongly influenced Fleetwood Mac.

The session featured Peter Green, Jeremy Spencer, Danny Kirwan, John McVie and Mick Fleetwood alongside Dixon, pianist and vocalist Otis Spann, harmonica player Shakey Horton, guitarist Buddy Guy and other Chicago blues figures. The recordings were originally issued in two Blue Horizon volumes in December 1969 and later appeared under titles including Fleetwood Mac in Chicago.

Otis Spann is the most common alternative people remember because of his prominent piano and vocal contributions, while Buddy Guy played guitar and Shakey Horton played harmonica. John McVie was Fleetwood Mac's own bassist, but the question asks specifically for the Chicago blues bassist who joined the session; that musician was Willie Dixon.