It’s not just that the business world is dominated by Windows-based computers. It’s because, at a number of large companies, Mac users literally can’t apply.

At American Express, for example, job seekers are greeted with this warning: “At this time, the system does not support Mac environments. If you don’t have access to a PC at home or work, please check out a local public library for Internet access, local Internet cafes, or the nearest government Work Force Center.”

The financial-services company, like a number of other well-known businesses, uses an outside talent management company, BrassRing, to manage its online job applications. However, the current generation of BrassRing’s software doesn’t support Mac browsers. A company representative said it hasn’t incorporated Mac support because clients haven’t been clamoring for it.

“However, we are seeing a gradual increase in demand,” said BrassRing spokesman Doug Jensen. “Therefore, BrassRing Enterprise 8, which is coming out in December, will be Firefox-compatible to support Mac users.”

For now though, online job seekers to a number of companies are likely to see problems. Target Stores and Sears are among the other companies that use BrassRing’s software to handle some or all of their online applications. While American Express offers a warning on its Web site, job seekers at Target and Sears may just get an error message or have their browser crash when using Apple’s Safari browser.

Despite the warning on the American Express site, some people reported being able to file an application online using Safari or Internet Explorer for the Mac, while others encountered glitches.

Representatives of Sears, Target and American Express were not immediately available for comment.

One job seeker, who asked to be identified only by his first name “Peter,” said that he found it unbelievable that a Fortune 100 company like American Express would shut out Mac users.

“Yes I know I could go to FedEx/Kinkos but I don’t have time for that,” said Peter, who didn’t want his full name used because he is still deciding whether to apply for the American Express job. “American Express has created a big barrier to hiring some candidates simply because we are Mac users.”

Although Windows-based computers dominate the markets for both business and consumer PCs, Apple has been steadily gaining share. The company saw its Mac unit shipments rise 48 percent in the third quarter. That gives the company a 4.3 percent share of the U.S. market, according to IDC, up a full percentage point from last year.