The Bill & Melinda Gates Foundation Trust and Cascade Investment, an asset management firm owned by Mr Gates, increased their combined holding in G4S to 3.2% last week by acquiring about six million more shares.
G4S, which operates in over 125 countries, suffered a blow to its reputation when it failed to provide a promised 10,400 guards for the London Olympics.
A profit warning last month triggered the departure of chief executive Nick Buckles, who was succeeded on June 1 by Ashley Almanza.
The Gates trust was set up in 2000 to fight poverty and disease. Its other UK investments include Diageo and Carpetright.