THE LARGEST STRIKE ever in South Africa is in its second week, and tension is building. Talks had opened between the black National Union of Mineworkers and the white-run coal and gold companies to discuss ways to reduce violence that has accompanied the strike. But yesterday the NUM broke off the talks at word that police were firing at strikers. It seems that several hundred thousand workers are out and major parts of an industry that provides most of South Africa's foreign exchange earnings are shut down.

Unions are the principal organized bodies legally permitted to blacks under apartheid -- a reflection less of white enlightenment than of a wish to secure an orderly work place. Still, it takes immense skill and courage to organize a work stoppage on any scale, even in the highly unionized mining sector. The unions lack resources and cannot provide strike pay; hardship facilitates recruitment of strikebreakers, and the companies have greatly preferred access to police and courts. In this strike, the miners, who earn a fifth of what white miners make, are asking for increases in the 30 percent range, plus other improvements. The companies -- such as Anglo-American Corp., internationally known as an outpost of South African progressivism -- come back with an offer of a smaller percentage increase. In addition, supervisory jobs are to be opened to black miners.

The items in contention in this strike are economic, but the strike goes to the essentially political question of how blacks will fight white oppression. This is why whites were reluctant to permit workers a body for collective self-expression in the first place. Now that the miners' strike is under way it is becoming a major test of power in a country where other forms of black assertion are rigidly proscribed.

For several years now, foreigners have been preoccupied by the issue of applying economic sanctions to South Africa to force internal change. It is often asked whether foreigners should be taking steps whose heaviest costs are borne by others -- by blacks in South Africa. In this strike it is South African blacks who are applying a sanction by withholding their labor and who, bravely, are themselves bearing the great costs.