Shares in French game maker Ubisoft were up 24% after the world's leading video game publisher, Electronic Arts, said it was buying 19.9% of the firm.
However Ubisoft said it would consider EA's stake-building as "hostile", until it received further details.
EA is buying a block of Ubisoft shares from Talpa Beheer BV, the investment vehicle of Dutch tycoon John de Mol.
Experts have valued the deal at between $85m (£43.6m) and $100m, making it one of EA's largest acquisitions.
EA said the stake did not allow it to exercise any control over Ubisoft, Europe's third-biggest independent video game publisher and the maker of recent hit game Prince of Persia: Warrior Within.
'Securing studios'
"We have no control over what Ubisoft's management chooses to do with the company," said EA spokesman Jeff Brown.
EA's stake would give it access to 18.4 % of the voting rights against 22.8 % held by Ubisoft's founders, the Guillemot family, Ubisoft said.
"Pending further information, we consider this operation as hostile," an Ubisoft spokesman told Reuters agency.
"We think this operation is aimed at securing the studios of Ubisoft that are ready to face the next generation of gaming consoles," he added.
American Football deal
The transaction needs to get US anti-trust clearance before it can be completed.
Ubisoft has a portfolio of more than 1,000 games titles and development studios in Canada, China, France and Texas.
It has enjoyed success with the Tom Clancy brand, with global hits such as Splinter Cell and Ghost Recon.
Ubisoft's most recent release in the UK, Prince of Persia: Warrior Within, is riding high in the games charts.
EA's titles include sports games include NFL Street.
Last week, it signed exclusive agreements with the National Football League and the NFL players' union.
It has a market capitalisation of $18bn, sales of $2.9bn and $2.5bn cash. Ubisoft has a market capitalisation of 300m euros ($402m).
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