December Dhladhla's hands were trembling as he drew from his pocket the mimoegraphed piece of paper that would disrupt his family's life.

"You are warned to take immediate steps to leave the prescribed area of Johannesburg within 72 hours," read the document addressed to Dhladhla's wife, Tsametse Virginia.

Dhladhla's wife had been "endorsed out" -- official terminology for being ordered to leave the all-back satellite township of Soweto adjacent to Johannesburg where she lives with Dhladhla and their two children. She must return to her hometown of Mafeking 180 miles away because she is not "qualified" to live hear Johannesburg.

"My heart is thinking very bad now, madam,: The 32-year-old suitcase maker told a visitor in broken English with an uncomprehending look on his face. "I can't stay like that. I don't want my wife going back. I want her here, madam, because I am born in Johannesburg."

The Dhladhla family was feeling the force of the most fundamental and most hated aspect of South Africa's apartheid system -- the "pass laws." This vast mesh of intricate laws, also called "influx control," regulates the movement and presence of blacks in "white" South Africa -- approximately 87 percent of the country -- and denies them the right to choose where to work and live.

According to official figures, 272,887 blacks were arrested for offenses relating to the pass laws in 1978, turning thousands of otherwise law abiding citizens into criminals and swelling South Africa's prison population.

Enforcement and administration of the pass system was calculated by one state more than $13.4 million a year, based on 1974 figures. In comparison, the current budget for black education is $215 million.

Because the pass system is a daily and blatant reminder to the 18.6 million blacks of their second-class citizenship in relation to the white minority of 4.5 million, their resentment of it repeatedly has been at the core of black political protest in this country.

In 1943 the now-outlawed black nationalist movement, the African National Congress, declared pass laws to be "enemy number one" of black people. Protests against the laws sparked periods of unrest in black communities in both the 1950s and 1960s. Their abolition is now a nonnegotiable precondition,according to most black leaders, for any meaningful dialogue with the government.

To the ruling white elite, however, influx control is the essential security measure that protects their life styles and exonomic activities. It stems from the principles of apartheid, whose aim is to fragment South Africa into 11 nation-states -- 10 black and one white.

Abandoning influx control would mean scrapping the apartheid system, under which all blacks eventually will become citizens of one of the 10 black states.

"Everything else would change at the same time and the whole political structure would fall down if the pass laws were abandoned," said Sheena Duncan, vice president of Black Sash, a private advice clinic run by white women for blacks.

The government argues that influx control, which has existed for more than a century here but which has been strenthened greatly under National Party rule in the last 30 years, is necessary to halt the city-bound migration of rural people and head off major urban social problems. They also say that influx control protects blacks already living in the cities from job competition and helps combat unemployment.

Critics say the system just contains unemployment in the rural areas.

The government argues that the pass system does not violate blacks' rights since they can live and work where they please inside one of the 10 black homelands. This argument is rejuected by blacks since the homelands are poverty-stricken, lack industries and make up only 13 percent of the land.

Sensitive to criticism about the pass system, the government claims to be initiating reforms to change it.

The passes, which all blacks are required to carry with them at all times, technically are bein phased out as part of the homelands scheme. But critics charge that the Pretoria government will merely use the new indentity documents that blacks must get from their states to restrict their movements and control their presence outside the tribal homelands.

In any case, the sustem of influx control is to remain in place. This system limits the right to reside permanently in an urban area to those blacks who were born in black townships such as Soweto and have lived there all their lives. According to official figures based on the 1970 census, this "right" applies to only 1.5 million blacks, a minority of the officially estimated 5 million urban blacks in the country. More recent estimates indicate that nearly 9 million blacks live in and around the urban areas of white-controlled South Africa.

Those blacks who are not officially "qualified" to live in cities permanently are either migrant workers, whose number is set at 1.1 million but is believed to be much higher, or "illegals." Migrant workers are recruited by government labor bureaus in their rural areas and must leave their families at home while they live in all-male hostels in urban areas as they work out their one-year contracts.

"Illegal" blacks have flocked to the industrial and urban centers to seek jobs, and many of them end up with employers who take advantage of their status to pay low wages. In Soweto alone illegals are variously estimated to number from 250,000 to half a million.

Unless a person is "qualified" he cannot legally get or change a job, apply to rent government housing, or set up his own business. If a black man, marries a woman who is "unqualified," for example, she does not automatically acquire the right to live in with her husband. She must apply for a permit to do so, which South African usually refuses to grant. This is the Dhladhlas' problem.

Because of natural population increase and economic growth, the "qualified" urban black population should rise. But the government has taken steps designed to make it gradually disappear.

The key to this effort is a law passed last year stipulating that children of "qualified" parents would no long automatically be allowed to live in the urban areas where they grew up if they were born after the independence dates of the ethnic "homelands" to which they nominally belong.

For most blacks the pass system becomes a reality at age 16, when they must apply for their passes or "reference books." Their fingerprints and biographical data are fed into computers in Pretoria, providing a ready source of information on the black pupulation. This information is a useful tool for the security police to help identify black dissidents and youths who have gone abroad for guerrilla training.

Whites are required to carry identification papers also. However, they only are fingerprinted if booked on suspicion of crime, and they are not obliged under pain of arrest to carry their papers with them at all times.

To enforce the pass system and catch illegals, police or labor officials periodically raid black homes in townships, factories, restaurants, department stores and even domestic servants' quarters in posh white suburbs.

These "pass raids" have intensified this year, prompting black leaders to warn the government of growing resentment.

Those who get caught in the pass raids end up in the pass courts where their cases are heard with remarkable dispatch. In the pass court of Johannesburg alone more than 7,200 cases were heard in the first three months of this year, according to one court prosecutor.

Thoko Tugwana, 22, worked in a popsicle factory for $14.40 a week and had just finished her lunch outside the building when police arrived. She told them her reference book was inside the factory, but since it was not on her person, they moved her into their van, she said. She spent the night in a cell where she said her blankets were filled with lice.

The next morning, she was among scores of women whose cases were heard. Those who were "illegally" in Johannesburg were fined or sent to serve prison terms of up to 60 days. Tugwana was luckier than most since her brother, alerted to her plight by a friend, came with bail money to free her so she could get her pass and prove she resided legitimately in Johannesburg.