In a big step forward for the clean-air bill, House and Senate negotiators reached tentative agreement on provisions requiring reduced smog-causing emissions from cars and sale of low-polluting "reformulated" gasoline in polluted cities.

The key parts of the agreement on motor vehicles would:- Give the Environmental Protection Agency discretion over how far to push requirements for reductions in smog-causing automobile tailpipe emissions after the year 2000.

- Require the sale of cleaner-burning "reformulated" gasoline starting in 1992 in cities with excessive carbon monoxide pollution. The gasoline would contain ethanol additives known to reduce carbon monoxide emissions.

- Allow states to adopt California's program for requiring sale of thousands of low-polluting cars and the use of such vehicles by companies with large commercial fleets.

Environmentalists gave a lukewarm reception to the agreement on motor vehicles, saying it would help clean up urban smog, but none too soon.