The Montgomery Board of Realtors is appealing a decision by Circuit Court Judge Joseph N. Mathias validating the "recapture tax" paid by home sellers in the county whose assessments are lower than the actual market value.

Last year the board, John C. Walker Realty Corp. and Bethesda risident Edward W. Schultz challenged the constitutionality of the recapture tax. But Judge Mathias ruled in April that it did not violate state law. The plaintiffs had argued that the tax is discriminatory because it is levied only on owners who sell their houses.

The tax was established to reclaim for the county - at the time of sale or transfer of real property - part of the revenues that would have been collected if the assessment relfected the actual sales price.

"Uniform assessments should be made so that tax obligations are shared equitably by all home owners," said realtors board president Jessie L. Peck.

He also said that, with a return to tri-annual assessments, the disparity between assessed value of a property and current market value could be substantial and could result in a larger recapture tax than when properties were formerly assessed annually.

The recapture tax works like this:

A house sell for $173,500. The tax obligation is figured on 40 percent of the selling price, minus an exemption of $18,000. That would bring the tax figure to $61,400. But the assessed value of the house was $50,800. So the seller is liable to a tax ($3.70 per $100 of valuation) on the difference - $10,600. That would make the recapture tax $392.20.