THERE IS GOOD REASON for the Senate Government Operations Committee to release Budget Director Bert Lance from his pledge to sell his 190,000-plus shares of stock in the National Bank of Georgia by Dec. 31. President Carter had asked Mr. Lance to get rid of the bank stock, and Mr, Lance had promised to do so this year, to avoid even the appearance of any conflict of interest. But the price of the stock, which Mr. Lance bought a few years ago at an average of over $17 a share, has dropped as low as $8.50 a shere in the last few weeks. One reason is that the bank, like many others, has reported losses stemming from bad real-estate ventures. Another cause of the decline, though, has been investors' knowledge that Mr. Lance's holdings, which amount to 21 per cent of the bank's outstanding shares, would be coming onto the market soon.

Thus meeting the deadline could cost Mr. Lance $1.6 million or more and could also inflict losses on other investors i n the bank. That would be a high price for a commitment to public service. Mr. Lance may be willing to pay that price if he has to, but the committee should grant him some relief by allowing him to keep the stock in a trust until a sale on less damaging terms can be arranged. After all, Mr. Lance has also promised to disqualify himself from policy matters involving regulation of banks. He did slip a bit last month, when he signed a letter to congressional conferees objection to Senate legislation that encourages financial institutions to invest in older neighborhoods. Strictly observed, though his promise to stay out of such matters should keep any real conflicts of interest from developing.

The question underlying all of this is how far public officials should be require to go to separate their private interests from their public jobs. In some cases divestiture is neccessary. Officials of regulatory agencies, for instance, are so deeply involved in the government of specific industries that if they kept related financial interests, even in blind trusts, they would have to step aside from their entire jobs. In cases such as Mr. Lance's, however, the official responsiblities are more diffuse. Especially where the divestiture is diffcult, financial disclosure and abstension from clearly related decisions should be enough. After all, no rules - no matter how stern - can guarantee impeccable conduct in government or any other field. As the administration seems to be learning, one can put too much emphasis on looking pure and, in the process, discourage people of character, competence and achievement from entering public sevice at all.