Software companies that make computer programs to help owners of personal computers file their income tax returns are finding a recent Internal Revenue Service ruling particularly taxing.

The agency now says that, if a tax program offers "substantive tax instructions rather than just mechanical assistance," the creator of that software is considered a "return preparer" and subject to a slew of possible IRS regulations.

"If there's an understatement in the return due to negligence" in the computer program, the tax agency could slap a $100 penalty, for every understated return, on the company, an IRS spokesman said.

"It makes software companies more liable than a host of other tax-preparer manuals, and we don't think that is a positive development," said Ken Wasch, executive director of the Software Publishers Association. "It creates an enormous responsibility on the part of the software developer and would have the effect of actually discouraging firms from creating very useful tax-management programs."

The ruling, originally issued last December and further clarified in an IRS release last week, also defines large-volume computerized tax-preparation services as "income preparers" subject to regulation.

More than a dozen companies make personal-computer tax programs -- ranging from "J. K. Lasser's Your Income Tax" software from Simon & Schuster to "Tax Preparer" from Howard Software Services in La Jolla, Calif. More than 100,000 of the tax programs have been sold, according to company statistics.

The ruling could be particularly damaging to companies seeking to market "artificial intelligence" software that would give taxpayers advice on what tax options -- such as shelters -- they should consider.

Some tax programs let users compare the amount of deductions they take with the amount claimed by others in the same tax brackets.

The software companies now stress that their programs are safe from IRS scrutiny because they simply calculate taxes rather than offer tax advice.

"As we understand the ruling, it relates more to programs that make decisions for you," said Bretta Brictson, a marketing executive with Howard Software. "Our program steers away from from that; we don't give tax advice, we're sort of like a calculator."

"Like any IRS ruling, I don't like it," said Michael A. Chipman, president of ChipSoft, which makes the Turbotax tax package. "What they're really getting at is programs giving advice -- it may remove some of our competition, though."

However, the IRS spokesman noted that using a computer for certain kinds of tax calculations could well qualify as "substantive tax instructions" and thus be considered a "return preparer."