Montgomery County teachers voted overwhelmingly yesterday to ratify a new three-year contract despite last-minute complaints that one portion of the agreement threatens longstanding seniority rights.

A spokesman for the Montgomery County Education Association, the union that represented 4,800 teachers during the past year of bitter contract negotiations, said 2,516 teachers, or 64 percent, voted in favor of the agreement, while 1,396 voted against.

The spokesman said the vote showed that teachers had a "far greater interest in their working conditions than ever before."

However, several teachers complained that the package of salary increases, totaling 11 1/2 percent over the first two years, was insufficient. Other teachers said they voted no because they did not favor a three-year agreement.

Teacher raises for the third year of the contract will amount to 100 percent of the average cost-of-living increase for the Washington area. The average annual teacher's salary in Montgomery is now about $28,000.

Other teachers sharply criticized one portion of the contract, which for the first time establishes a specific guideline affecting seniority rights during involuntary transfers.

Richard Jaworski, 43, a vocational arts teacher at Springbrook High School in Silver Spring, said he voted against the contract because the new clause states that in involuntary transfers, the county's "balanced staffing" policy "will take precedence over the consideration of seniority." The contract says that each school staff "should be appropriately balanced in terms of sex, race, ethnic background, age and experience."

"That's an attack on the seniority system" said Jaworski, a 17-year veteran of the county schools.

MCEA Executive Director Walter W. Rogowski disputed Jaworski's contention, saying that there was little difference between the new language and a similar statement of principles that appeared in the 1977 contract.

Said Rogowski: "What we're saying [to MCEA members] is: 'Just in case you folks missed it, the principle applied in the past to involuntary transfers and we're putting it in here."

In the past year, there were 645 involuntary teacher transfers in Montgomery, due largely to fluctuating school enrollments.

In neighboring Prince George's County, teachers have had language virtually identical to Montgomery's in their contract for the past decade, a spokesman said.