Roche has launched a $43.7 billion (£21.9 billion) takeover offer for biotechnology company Genentech, in which it is already the majority stakeholder.
Swiss drug-maker Roche is offering $89 (£44.6) per share in cash, an 8.8 per cent premium on the shares from the previous day. Roche bought 55.9 per cent of Genentech's shares in 1990.
Roche chairman, Franz Humer, said: "The transaction will create a unique opportunity to evolve Roche's hub-and-spoke model into a structure that allows us to strengthen the focus on innovation and accelerate the search for new solutions for unmet medical needs.
"Combining the strengths of Roche and Genentech will create significant value and result in benefits for patients, employees and shareholders."
According to Roche, Genentech will continue to operate as an independent research and development centre within Roche and will retain its staff.
The combined group would become the seventh largest US pharmaceuticals company in terms of market share, generating more than $15 billion (£7.5 billion) in annual revenues and employing around 17,500 pharmaceutical employees in the US alone, Roche said.
Annual savings could be between $750 million (£375.6 million) and $850 million (£425.7 million), the company has estimated.
Genentech was founded by venture capitalist Robert Swanson and biochemist Herbert Boyer in 1976, and has been at the forefront of the rapidly-growing field of biotechnology.
Medicines developed by Genentech include breast cancer drug Herceptin and age-related blindness drug Lucentis.