Proposition 1, an antibusing measure approved by California voters in 1979, was held constitutional yesterday by a state appeals court.

If allowed to stand, the unanimous ruling could end mandatory busing in Los Angeles public schools in the middle of this school year. But there are several more levels of appeals.

The ruling does not go into effect for 30 days and at that time the American Civil Liberties Union and other parties in the long legal wrangle over school desegregation in the nation's second-largest school district can appeal the order to the state Supreme Court.

The three justices said they found no intentional segregation in the Los Angeles City School District. Threrfore the district would not be required, under the common interpretation of Proposition 1, to continue mandatory busing to achieve integration.

"We conclude that the racial imbalance and segregation which existed in many schools in the district and the board's actions in relation thereto did not constitute a violation of the equal protection clause of the 14th Amendment, as interpreted by the United States Supreme Court, in that racial imbalance and segregation did not result from board acts performed with segregative intent and discriminatory purpose," said the justices.