Wal-Mart Stores Inc. (WMT) has expanded its prescription drug program to companies beyond Caterpillar Inc. (CAT), where it has been running a pilot program since September.

The world's biggest retailer has signed other companies to the program, said spokeswoman Christi Gallagher, in the company's first public comment about the expansion.

Wal-Mart declined to name the additions, saying the information was proprietary. Wal-Mart had used Caterpillar as a banner to bring attention to its aspirations to define itself as a supplier of prescription medicine to businesses.

Wal-Mart's comments came Wednesday after Caterpillar said it would be working with Walgreen Co. (WAG) to offer prescription medicines to Caterpillar's 70,000 employees, retirees and their dependants as of Jan. 1, 2010.

Caterpillar also said it was signing a two-year contract with Wal-Mart, effective the same date, to take the program from its pilot stage and continue it.

-By Karen Talley, Dow Jones Newswires; 212-416-2196; karen.talley@dowjones.com