The distinctive new red, white and blue diplomatic license plates may make it easier to spot foreign officials parking in front of fire hydrants, but they are also coded to help the FBI's counterintelligence agents keep track of possible spies.

Two letters on the side of the new plates identify the foreign mission to which the plate has been issued, and counterintelligence agents have been furnished with a wallet-sized card that identifies the codes of 18 countries.

This list of so-called "criteria" countries includes the Soviet Union (SX) and the rest of the Iron Curtain countries, Cuba (DC) and Nicaragua (QU), China (CY), Vietnam (LD) and North Korea (GQ). Also listed are four countries suspected of supporting terrorism, Iran (DM), Iraq (TS), Libya (FM) and Syria (AQ). The code for South Africa is also listed, and the reason for that may be "reciprocity" -- the South African intelligence service keeps a close watch on American diplomats in that country.

Allies such as Great Britain (WZ), France (DJ) and Canada (TG) aren't on the special watch list. State Department officials who issue the tags, and the FBI officials who keep an eye on them, are reluctant to discuss how countries make the "criteria" list.

The new plates are the first to have country code designators, and officials cite several reasons why the change was made. The old DPL plates used in the Washington area weren't that easy to spot, and officials note that it simply isn't possible for our agents to follow all of those suspected of being their agents.

The new system also replaces and centralizes what was a hodge-podge of more than 18,000 diplomatic plates from 28 jurisdictions around the country. Even some American citizens who were "honorary consuls" to foreign countries had diplomatic plates in those days -- a good way to avoid parking tickets.

President Reagan said in a speech last week that "we need to reduce the size of the hostile intelligence threat we're up against in this country." He noted that there are 2,500 Soviet bloc officials in the United States and from 30 to 40 percent of them are known or suspected intelligence officers.

There are more than 1,000 Soviet diplomats in the United States, including nearly 300 attached to the embassy here. Using the president's estimate, that means there may be 100 agents of the KGB or the GRU, Soviet military intelligence, in the Washington area, without counting its Iron Curtain allies.

Travel restrictions limiting members of some missions to staying within a 25-mile radius of the White House help keep track of the Soviets and Cubans. That circle, however, includes some of the most sensitive areas around, from the Pentagon to the Central Intelligence Agency to the National Security Agency, home of the U.S. code breakers, at Fort Meade.

In fact, Soviet bloc agents have been known to sit in cars on Rte. 123 in McLean and write down the license tag numbers of employes going to work at the CIA.

James E. Nolan Jr., a former FBI counterintelligence officer who runs the Office of Foreign Missions at State, said the new plates are a help to the FBI, military security officers and defense contractors trying to keep tabs on espionage. But the net can only spread so wide, officials say.

Poolesville in Montgomery County, for instance, wouldn't seem like a hot spot for a Soviet spy to be visiting. But it is just inside the 25-mile limit, and when FBI agents were tailing accused Soviet spy John Walker near there in May they spotted Aleksey G. Tkachenko, a vice counsel at the Soviet Embassy, who wasn't being followed at the time.

Walker was arrested after allegedly leaving a bagful of classified documents in the woods near Ferry and Partnership Roads. One source said FBI agents ran into Tkachenko, who is said to be a KGB agent, by luck. Tkachenko didn't stop at the so-called "dead drop" site to pick up Walker's package, and left the country a few days later.

In 1983, another Soviet diplomat was picked up by FBI agents at another dead drop site a short distance from the one Walker allegedly visited. That diplomat, a military attache, was declared persona non grata and told to leave the country.

A few weeks ago, the lack of resources for full-time surveillance apparently cost the FBI a chance to catch a suspected spy in a compromising position. Col. Sergei Smirnov, the air attache at the Soviet military mission -- and a GRU agent according to FBI sources -- plowed into a car on the Rock Creek Parkway. The Park Police report said when Smirnov got out of his car that Saturday at 5 p.m. "he could not walk on his own, was babbling incoherently and was in a general state of severe intoxication."

A knowledgable source said Smirnov wasn't being tailed at the time, and the Park police didn't call the FBI for several hours. So the bureau missed an opportunity. Nolan said Smirnov's driving privileges have been revoked.

From now on Smirnov might have to take Metro. A Soviet isn't supposed to rent a car, or make airline or train reservations to see the country, without going through his office first, Nolan said. If they want to rent a car, the State Department will arrange one, complete with chauffeur, he added. He wouldn't say if the driver carries an FBI badge.

Soviet spies can take a bus or subway, of course. And as unglamorous as it sounds, one Soviet spy, Yuriy P. Leonov, a GRU agent, used to park his car at Tysons Corner and take the bus to McLean during 1983 in an apparent effort to avoid detection. But he was going to visit an American who edited technical publications and was cooperating with the FBI. Leonov was caught accepting carefully prepared classified documents, declared persona non grata and kicked out of the country.

The new diplomatic tag system cuts the old number of 18,000 plates back to 14,000 by limiting plates to foreigners paid by the sending country and members of their family, Nolan said.

Nearly half of the registered diplomats are in the Washington area and, in a resolution of an argument that has sputtered for years, they are now required to have liability insurance. Newcomers to the foreign missions here pick up their new plates from the building at 3005 Massachusetts Ave. that used to be the Embassy of Iran.

Straightforward license tag designators such as (SU) for the Soviet Union and (UK) for Great Britain aren't used because of concerns about vandalism, Nolan said. But he said there have been few complaints about the system from foreign embassies. The Soviets have used a similar system for years.

Nolan said the codes also are used because if the country designators are widely known "we'll start getting kids trying to play Sam Spade and follow them around."