24 percent stake in ADT Ltd. to Western Resources Inc. for $215 million in cash, or $14 a share.

The Burlington, Ontario-based environmental and transportation services company now owns 30.8 million shares in ADT. ADT, based in Bermuda, provides electronic security services.

Laidlaw also granted Western Resources, a Topeka, Kansas-based power utility, an option to buy its remaining 12 percent stake in ADT before an exercise deadline of May 15, 1997.

The purchase price would be at $14 a share, or higher if the shares trade higher at the exercise deadline. ADT shares were up 3/8 at 13 7/8 in late morning trading on the New York Stock Exchange.

Laidlaw's Class B shares were up 1/4 at 10 1/8 on the New York Stock Exchange.

The Class B shares were up 1/2 at 13 7/8 in Toronto. Western Resources's stock was down 1/8 at 32 7/8.

"This sale represents a significant step for Laidlaw in its efforts to focus on its core business,'' said James Bullock, Laidlaw's president and chief executive.

He said the company, founded by Bermuda resident Mr. Michael DeGroote, will use the cash to retire debt. At August 31, Laidlaw's long-term debt was $1.67 billion. Laidlaw had fiscal 1995 profit of $132.8 million and revenue of $2.52 billion.

If Western Resources chooses not to exercise its option to purchase the 15.4 million ADT shares, Laidlaw will sell the stake elsewhere or use the shares as a currency in the retirement of its $280 million ADT-linked 6 percent debenture issue due January 1999.

Laidlaw had taken a $141.6 million charge to write down its investment in ADT in 1993 "in order to reflect the company's determination of a decline in realisable value''. The company began accumulating ADT shares in April 1989.

The company is North America's largest school bus and ambulance services provider in the US and Canada. It is North America's third-largest recycler and manager of household, commercial and industrial waste.

Laidlaw recently acquired CareLine Inc., an ambulance services provider based in Irvine, California, for $330 million, including $185 million in debt.