Prior to regulation, these bus lines were listed in a feature in the publication "Bus Transportation", with their May 1922 issue. But unfortunately the actual operators were not identified. However, the October 1923 issue of that publication had a feature on the bus operations of The Milwaukee Electric Railway & Light Co. That company operated all local electric railways in the Milwaukee area, including interurban lines to outlying areas. That company's first bus line was introduced in summer 1919, between Burlington and Lake Geneva.
The Milwaukee Electric Railway & Light Co. eventually operated nearly all bus routes in Milwaukee and southeastern Wisconsin. At the time, the streetcar and interurban lines in most other cities were operated by the local electric utilities. And presumably, many of the early bus routes operating out of those other cities were also operated by those utilities. Most of those utilities also eventually operated the local buses, which replaced their streetcar systems.
Appleton-Green Bay
Appleton-South Kaukauna
Appleton-Menasha-Neenah
Ashland-Park Falls
Berlin-Eureka-Omro
Berlin-Oshkosh
Burlington-Lake Geneva
Eau Claire-Augusta
Eau Claire-Chippewa Falls
Eau Claire-Menomonee
Eau Claire-Mondovi
Fond Du Lac-Madison
Green Bay-De Pere
Janesville-Edgerton
Kenosha-Racine
Madison-Mendota
Madison-Middleton
Manitowoc-Two Rivers
Milwaukee-Hartford
Milwaukee-Waukesha
Milwaukee-Menomonee Falls
Milwaukee-Oconomowoc
Milwaukee-Muskego Center
Oshkosh-Ripon
Oshkosh-Winneconne
Oshkosh-Green Lake
Sheboygan-Kiel
Superior-Duluth
Wausau-Mosinee
In addition, local bus routes existed in Kenosha, Manitowoc, Merrill, Milwaukee, Stevens Point, and Superior.