More than a year after she was injured in an automobile accident in Arlington, and just before her lawsuit to recover damages from the other driver was to go to trial, Robin Anne Lynn learned that the woman she was suing was employed as a secretary at the Brazilian Embassy.

Geico, the embassy employe's insurance company, also learned about her special status about the same time, and its attorney promptly got Lynn's lawsuit thrown out of court on grounds that its policyholder had diplomatic immunity.

Lynn spent three weeks in Arlington Hospital after the accident and since has fully recovered. Both women claimed they had a green light when their cars collieded at the intersection of U.S. Rte. 50 and Pershng Drive. Mrs. Mendes was driving her husband's car, and was not on embassy business, according to Lawrence Blumberg, Lynn's busband. Neither Mrs. Mendes nor her attorney could be reached for comment yesterday).

Blumberg, who is a lawyer, said both women sued for large amounts, but that actual unrecovered damage suffered by his wife came to about $6,000, including loss of their 1972 Opel, medical bills not covered by insurance, and salary lost from her job at the Smithsonian.

Blumberg, testified yesterday in behalf of a bill sponsored by Sen. Charles McC. Mathias (R-Md.) that would make it possible for persons such as Lynn to recover damages from diplomats.

Blumberg told Sen. Howard M. Metzembaum (D-Ohio) chairman of the Subcommittee on Citizens and Shareholders Right and Remedies, that the question of immunity for the woman, Tethys Coelho Mendes, had not been raised during a year of suits and countersuits arising from the accident on Nov. 24, 1974.

Mrs. Mendes' lawyer brought up the subject after depositions were taken and "it apparently became clear that her (Mrs. Mendes') case was weak," Blumberg said.

Stacy L. Williams, Geico's assistant vice president, who also testified, said he did not know the details of the Lynn case, but defended Geico's action in getting the case dismissed.

"It would be absolutely foolish to waive diplomatic immunity," unless the policyholder agreed, Williams said.

"You would not have been foolish morally," Metzembaum responded.

"We do not pay moral claims," countered Williams.

"Why did you take this woman's (Mednes) money?" asked an angry Metzenbaum. "Why did you write the insurance? You never expected to pay out a penny. Isn't that immoral?"

Williams said it was standard procedure for Geico employes to determine at the time a policy is written if embassy employes plan to waive immunity if involved in an accident. If they are not, Williams said, Geico tells the diplomats about "certain circumstances in which you might not need insurance."

Shaking his head in disgust, Metzenbaum said "I'm not taking about paying money for moral claims, but about the moral issue 8f raising the defense" of diplomatic immunity.

Williams said after the hearing that Geico claims investigators probably should have uncovered the fact that Mrs. Mendes was employed by an embassy during their investigation of the accident.

Metzenbaum asked another witness, Howard B. Clark, former assistant to the Federal Insurance Administrator, if selling policies to diplomats who then claim immunity results in "windfall profits" for insurance firms.

"Yes, and it offends every principle of insurance law," Clark said.

Mathias' bill would require a state to refuse to issue a driver's license or behicler egistration card to an embassy employe who could to not show proof of liability insurance. If also would allow injured parties to sue the diplomat's insurance company, and preclude the insurance company from hiding behind a diplomat's immunity.

Mathias, who was one of the witnesses yesterday, has introduced a second bill that is identical to one proposed last year in the House by Rep. Joseph L. Fisher (D-Va.).

The House passed an amended version of Fisher's bill last July. It would keep full immunity for embassadors and other top-ranking diplomats, but withdraw the protection, in varying degrees, for other lower-ranking personnel.

Fisher todl Metzenbaum his interest in diplomatic immunity "was aroused by a tragic accident in my district two years ago." He said the House bill would make about 4,000 of 6,000 embassy employes in the Washington area subject to normal civil suits for their unofficial acts, including auto accidents, apartment leases, bad checks and personal contracts.