Pfizer is abandoning plans to buy AstraZeneca, a British-Swedish drug company.
AstraZeneca fiercely resisted a £69 billion bid by the American pharmaceutical group. It rejected a final offer from Pfizer earlier this month, saying the proposal undervalued the company.
Many investors view the offered price of £55 per share as too low.
The BBC says the decision ends what would have been the largest merger in the industry's history.
Pfizer had until Monday evening to make any further move under a strict timetable allowed for takeovers of public companies.
The same rules mean Pfizer must now wait six months before making a fresh bid for AstraZeneca.