Google’s advertising system is under investigation by the US Justice Department.
The leading search engine said that it had put aside $500m (£306m) to settle any potential charges.
The investigation will look at how Google’s automated advertising system treats some unnamed advertisers.
The European Commission is carrying out a similar study.
It was said that the putting aside of $500m, its net profit for the first three months of this year would be less than first reported in April, falling to $1.8bn from $2.3bn.