(FORTUNE Magazine) – Say farewell this summer to tie-throttled necks and blazer-burdened shoulders. More corporate types are exchanging their formal business suits for casual khakis and cotton culottes. General Dynamics' tank division, H.J. Heinz's + world headquarters, and others have joined the likes of Allstate Insurance and PepsiCo as places that let employees leave their corporate battle dress at home at least part of the time. Look for other companies to follow this lead come June 17, the fourth annual Casual Day fundraiser for United Cerebral Palsy Associations. Employers collect donations for the charity from employees, who in exchange get to dress down at work. Over 12,000 companies are expected to participate this year, up from 6,600 in 1991, benefiting the cause by about $2.5 million. Many companies use Casual Day as a takeoff point to further loosen apparel restrictions, which bodes well for Levi Strauss, a sponsor of the event. The haberdasher, which already sells Dockers and other nonjean clothes, wants even more of the casual market. The company offers an informational kit containing various tips for top managers. They could, for example, stage a fashion show as a way of demonstrating not only how to go casual -- but also where to draw the line. The NPD Group, a market research firm in Port Washington, New York, has found that morale increases in 61% of workers who are permitted to unwind sartorially. Says Janis Lockridge, a manager at American President Cos., an Oakland shipping company that has a relaxed dress code: " Casual dress gives you freedom to be more individual. Now I've got the flexibility to wear things that actually look a little bit more feminine."