The Commodity Futures Trading Commission yesterday levied its biggest fine ever -- $200,000 -- on a St. Louis brokerage house for its part in a 1978 scheme to manipulate silver prices.

Clayton Brokerage Co. agreed to pay the fine without admitting or denying it was responsible for the actions of a group of former employes -- including one-time Clayton president Horace Salmon -- who face further charges from the CFTC.

The CFTC complaint accused Clayton of cheating and defrauding customers, hiding the true ownership of commodity accounts, improperly handling customers' money, failing to file federal reports and failing to supervise its employes.

Federal investigators charged that a group of Clayton employes concocted a scheme to force up the price of silver and make quick profits for themselves. aThe Clayton brokers secretly set up a company in the Cayman Islands to buy silver futures, the CFTC charged.

Then they allegedly persuaded a foreign investor to ask for delivery on a large number of silver futures contracts and advised many of their other customers to buy silver at the same time on the same day.

Knowing a lot of orders were coming and silver prices probably would jump dramatically, the Clayton officials sent through orders for their own accounts before they made the purchases for their customers. After those orders 6,000 contracts for the foreign buyer, and prices on the Chicago Board of Trade jumped to what the CFTC called "artificially high levels."

Participants in the plot, the CFTC charged, included Donald Dial, former manager of Clayton's Dallas office; Dial's son Scott; James Jarboe, former manager of Clayton's Atlanta office; and Norman Kirst and Ronald W. Harrison, two more Clayton employes.

The brokerage said yesterday it had "always clearly prohibited activity of the type alleged" and no longer had any of the other defendents on its payroll. Clayton said it agreed to the record fine after deciding "it would be more productive" to settle the case than to fight it in court, which could take years.