John Crutcher {op-ed, Oct. 22} argued that U.S. sanctions against South Africa have been counterproductive; he said that thousands of black workers have lost jobs while hasty corporate departures have turned a hundred whites into millionaires -- that sanctions have actually contributed to further power for the white rulers.

There is a germ of truth in what Mr. Crutcher says. Indeed, the sanctions Congress has so far enacted have affected -- if not focused on -- black workers. Sanctions enacted in 1986 were limited to such labor-intensive South African industries as textiles, coal, uranium and steel. Few restrictions on American exports to South Africa were included. Although blacks have said they are willing to bear the pain of sanctions to overthrow the tyranny of apartheid, the United States could add sanctions that focus on the white minority rulers and have a negligible effect on blacks.

A large proportion of South Africa's gross national product is accounted for by technological and industrial exports to South Africa. Cutting off that trade would have a significant effect on the white rulers and their government. Although computers and high-tech capital equipment exports to South Africa go almost exclusively to whites, neither President Reagan nor most of his congressional critics are considering adding such products to the sanctions list.

IBM sales alone, for example, amount to about $200 million per year, nearly one-half a percent of South Africa's GNP. In a letter published in the Los Angeles Times last year, IBM management acknowledged that in South Africa "very few of our customers are black." Stopping computer sales is one kind of sanction that would focus significant pressure directly on the white minority but have insignificant effects on blacks.

Furthermore, computers and high-tech capital equipment help reduce South Africa's dependence on unskilled labor. The addition of high-tech products to the sanctions list would thus sharpen the pressure for change without hurting job prospects for unskilled black workers. I wonder if Mr. Crutcher would argue against such a sanction?

JAMES LEAS Bethesda