The largest, and perhaps only, legal supply of Laetrile for cancer patients on the East Coast was seized by federal agents today in an orderly raid on a small corner drug store on the northeastern fringe of Baltimore.

Three agents from the Food and Drug Administration entered the back room of Henderson's Pharmacy shortly after 1 p.m., handed pharmacist Robert Henderson a receipt, and walked off with several boxes full of the controversial anti-cancer drug which they claimed he was distributing illegally. They then followed Henderson to his home in suburban Rosedale where another supply of Laetrile was siezed.

The entire stock of the apricot-derived drug was said to be worth about $35,000.

The federal agents literally arrested the Laetrile tablets and amputes - but not their owner, Henderson - and impounded them at the federal court house in downtown Baltimore. The government, in a complaint signed by U.S. Attorney Jervis S. Finney, charged that Henderson had been selling Laetrile to persons who were not legally allowed to receive it. Laetrile has been banned by the FDA.

Since a federal court ruling in Oklahoma last May, however, thousands of terminally ill cancer persons have been receiving the drug. These patients must file affidavits certifying their condition, and they are allowed to take the Laetrile only after it has been inspected by the government.

The complaint against Henderson, which will be heard in federal court next week, alleges that he was distributing the drug to persons who had not filed the affidavit.

Henderson, 37, whose store displays John Birch Society literature deerying the encroachment of government into private enterprise, said the FDA had no grounds to seize his Laetrile supply. "They'd do anything to get it," he said. "I'm just glad it (Laetrile) is at the courthouse now. If those (FDA) people had it, they'd destroy it."

This feeling was shared by some 40 Laetrile supporters who shuffled in and out of Henderson's modest establishment for most of the morning waiting for the well anticipated federal action. "I don't understand those people," said one man from Baltimore County. "Here we've got a government that wants to legalize marrjuana and they won't let my nephew take a drug that's going to ease his pain."

This man, along with many of his pro-Laetrile comrades, sneered and hissed as the federal agent walked off with the boxes of Laetrile. But there were no untoward incidents and no arrests. "That's not to say that those people were not burned up," Henderson said later. "I had to handle the situation very carefully."

The impoundment of the drugs today came at the end of a 30-hour drama that at times found the federal agents acting in cloak and dagger style. It began at 5:15 yesterday afternoon, when two agents and a U.S. marshal approached Henderson at his drug store and demanded that he turn over his Laetrile supply.

"I refused," explained Henderspn, "I told them that I needed a receipt from them and that I also had to talk to my lawyer first."

As a result of this refusal, the agents undertook an around-the-clock surveillance action. Two agents followed Henderson home last night, then trailed him in a car all the way up to Hereford, Md., a town near the Pennsylvania border, where Henderson conferred with his attorney, Edward Mackie. "Of course," said Henderson. "I had to give them directions to follow me."

Meanwhile, other agents were stationed back at the drug store on Harford Road, where they kept an allnight vigil. Alarmed neighbors twice called the police, believing that the agents were in fact prowlers.

"It was kind of ridiculous," said Henderson, "but in a way it was all right. My store has been broken into six times. So at least it was well protected last night."

The agents were there again, outside Henderson's home, when he awoke this morning. "I went out to them and told them that my wife and two kids were getting upset about it. I told them my family was going over to my father's house, that they could inspect my wife's car before she left so that they wouldn't have to follow her around. There was no way that she was going to slip off with some Laetrile. But they followed her anyway.

"Then some others came down to the store with me. They stood around with all of my Laetrile supporters. After my attorney worked out an arrangement with the government that the Laetrile would be kept at the courthouse I turned it over to the agents. They are gone now, thank God."

Henderson, a graduate of the University of Maryland School of Pharmacy, said he became interested in Laetrile three years ago when he met the drug. "He seemed to be getting a man who had cancer and was taking better as a result of it," said Henderson. "Or at least feeling better, I studied everything I could about it and eventually got into it as part of my business here."

Henderson estimated that he distributes about $50,000 worth of Laetrile each month. "As far as I know," he said, "I'm the only pharmacist on basis. The black market, now that's another matter. The black market must be delighted that I'm temporarily out of business."