The government's new system for preventing contamination in processing plants, known by the acronym Haccp, was implemented Monday for the 312 largest meat and poultry processing plants that account for 75 percent of livestock slaughtered in the United States.

The ''Hazard Analysis and Critical Control Points'' system will be phased in over two years in the remaining 6,100 plants.''We definitely have our work cut out for us, as there are many disturbing pitfalls and apparent weaknesses,'' Randy Wurtele, Western president of the National Joint Council of Food Inspection Locals, said in a letter on the union council's Internet site.

Under the new regime, plants install their own facilities' preventive measures to reduce E. coli and salmonella bacteria and improve sanitation.

Agriculture Secretary Dan Glickman said the system is a ''revolutionary improvement'' over the old approach's reliance on sight, touch and smell.

''Rather than catching problems after they occur, we will now focus on preventing problems in the first place,'' Mr. Glickman said.

Haccp systems involve identifying points in a processing plant where contamination is most likely to occur and finding methods to combat it. Each plant can design its own Haccp system but must meet certain standards.

Some of the 7,500 federal inspectors on the front lines say relying on company workers to keep records on how well the systems operate places too much faith in the honesty of corporations out to make a profit.