The deal to buy St. Jude raises doubts that Abbott will go through with its purchase of diagnostics company Alere, which is under federal investigation for its sales practices.
Abbott said on Thursday it would take on or refinance about $5.7 billion of St. Jude's net debt.
St. Jude shareholders will receive $46.75 in cash and 0.8708 Abbott shares, or about $85 per share. This represents a 37 percent premium to St. Jude's Wednesday closing.
St. Jude's shares were up 27 pct at $78.82 in premarket trading. Abbott shares were down 5 percent at $41.70.