- Charles Quigley
- Duncan Richards
- Linda Stirling
- Diana Farnsworth
- Clayton Moore
- Louis Ashe
- I. Stanford Jolley
- Dr. Blackton [Ch. 11]
- Kenne Duncan
- Dr. Chambers [Chs. 1-2, 8]
- Forrest Taylor
- Prof. Van Wyck
- Emmett Vogan
- Anderson [Chs. 1-4]
- Sam Flint
- Maxwell
- Joseph Forte
- Prof. Parker
- Stanley Price
- Count Fator [Chs. 11-12]
- Wheaton Chambers
- Wilson [Ch. 1]
- Tom Steele
- Henchman Stricker [Chs. 4, 7, 9]
- Dale Van Sickel
- Henchman Harte [Chs. 6-7]
- Rex Lease
- Bain
- Fred Graham
- Snyder [Chs. 1-2] / Zane [Ch. 8]
- Bud Wolfe
- Cross - Apartment Thug [Chs. 11-12]
- Rod Bacon
- Armored Car Guard [Ch. 3]
- Virginia Carroll
- Nurse [Chs. 8-9]
- John Daheim
- Police Driver [Ch. 3] / Relief Guard [Ch. 9] (uncredited)
- Carey Loftin
- Lab Guard #1 [Ch. 9]
- George Magrill
- Henchman Fiske [Ch. 3]
- Jimmy Noel
- Clerk
- Eddie Parker
- Henchman Sherman [Chs. 1, 8]
- Rose Plumer
- Mrs. Malloy [Ch. 11]
- Loren Riebe
- Truck Henchman [Ch. 6]
- Eddie Rocco
- Kell
- Dick Rush
- Police Inspector [Ch. 12]
- Bill Scully
- Richards' Assistant
- Duke Taylor
- Lab Henchman [Chs. 7-8]
- Ken Terrell
- Warehouse Thug [Chs.10-11]
- Robert J. Wilke
- Scott - Armored Car Driver [Ch. 3]
- Bill Wilkus
- Cole the Pilot [Chs. 5-6]
- Bill Yrigoyen
- Lab Henchman [Ch. 2]
- Joe Yrigoyen
- Janitor-Thug [Ch. 1]
