Genentech announced on Thursday that it would spend up to $371 million on retention bonuses to prevent its employees from leaving because of the possible acquisition of the company by its majority shareowner, Roche.

Genentech included the detailed plan in documents filed on Thursday with the Securities and Exchange Commission.

The company said the retention plan would encompass practically all of its 10,700 employees. Its chief executive, Arthur D. Levinson, would be entitled to a bonus of $8.7 million.

Roche, which owns 55.9 percent of Genentech, proposed last month to buy the rest of the company for $89 a share, or $43.7 billion. A committee of independent Genentech directors rejected that offer last week but said it would mull over a higher price.

The retention bonuses will be paid irrespective of whether the merger goes through or not, and are in lieu of 2008 stock option grants. Genentech said that the bonus program would cost it about 22 cents a share in earnings, spread over three years. The stock options would have cost 23 cents a share over six years.

The bonuses would be paid by June 30 if the acquisition is not concluded by that time. If the merger is accomplished by then, full bonuses would be paid when the deal closes if stock options remain unvested. If all stock options become vested, employees would receive half the bonus after finishing of the deal and the half one year later if they remained at Genentech.