Representatives of two black organizations in Prince George's County urged the County Board of Education last night to adopt a goal of awarding 30 percent of the school system's expenditures to minority-owned businesses.

A proposal, introduced last night by board members Thomas Hendershot, Sarah Johnson and Barbara Martin, would set such a goal and establish preferences for county-based firms in the school's annual procurement budget of nearly $40 million.

"The board has no knowledge of the percent of its business which it is doing with minorities," said June White-Dillard, from the National Business League of Southern Maryland, established in 1900 for black businesses. "In a county where the population is at least 40 percent black, that can no longer be tolerated."

Superintendent John A. Murphy estimated that the measure would cost as much as $500,000, leading him to express concern that it would take money away from the instructional program.

The proposal, which will be voted on in coming weeks, was also supported by Linwood Jones, president of the Black Democratic Council, a group concerned with education, politics and economic development.

The school system is now covered by a state policy requiring that 20 percent of school construction funds be awarded to minority businesses. But according to the sponsors of the county measure, the state policy affects less than 1 percent of all expenditures by the school system.