President Carter decreed yesterday that as it was for Caesar's wife so it must be for Carter's family -- it must be above suspicion, at least as of Oct. 1, 1980.

In fulfillment of a promise made after the controversy erupted about Billy Carter's relationship with Libya, the president issued guidelines telling government officials summarily to reject all business approaches from his family members, except in extraordinary cases.

Carter said that when a member of his family seeks to do business with the government, whatever the merits of the proposal, "many will believe, without any other evidence, that the governmentt's response was influenced by the family member's status as such."

"The president has therefore cautioned family members from making such proposals or requests, and urges all government employes not only to reject all such proposals and requests, but to report their occurrence to the head of the department or agency, who should advise the counsel for the president," Carter said in a memorandum to heads of federal agencies and departments.

Government employes should also "apply a strong presumption" against disclosing information of economic value not available to the public to his family members, Carter said.

Although his restrictions on aiding family members were broad, the president defined his family rather narrowly in his guidelines.

Carter said his parents, brothers, sisters and children and the spouses of his brothers, sister and children shoould be counted as family. More distant relations are not covered.

"While it could be arguued that members of the president's family have the same right as any other citizen to have the government engage in discretionary dealing with them, this is a right that is best relinquished during the president's incumbency," Carter said.

His guidelines leave murky the question of how a government employe should respond to an approach from a business with which a member of the president's family is associated if the family member remains in the background.

Carter said the guidelines do not apply to such a business "so long as the family member does not participate in any way, and the family member's association is not otherwhise exploited, in the entity's dealings with the government."

The guidelines also permit approval of a family member's proposal "in extraordinary cases where the responsible [government] employe believes the proposal or request should be approved."

An "extraordinary" case, the guidelines said, would be when the family member's business relationship with the government began before the president was elected and has not been exploited as a result of his election. The agency or department head must sign off on approval of such cases.

The guidelines focus on the discretionary award of government contractss and licenses, which can have enormous financial value.

Billy Carter is only the most recent of relatives of several presidents to have been seen by some as attempting to trade on his family tie. The president's brother was not seeking in a government contract or license, but was attempting to arrange a business deal with Libya that some have alleged would not have been available to him had he not had a White House connection.

Billy Carter profited by receiving $200,000 in two installments on what he said was to be a $500,000 interest free loan from Libya.

He said yesterday that he would begin paying back the loan with a first payment in January and a second in March. On the NBC "Today" show, Carter said he expected no problem in making the payments.

The president's guidelines distinguish two areas of family activity in addition to possible business dealings with the government.

In all, Carter said, "the primary responsibility to avoid any impropriety of course rests on the president and the members of his family."

In one area -- when family members are responding to one of Carter's frequent requests that they act as official U.S. representatives -- the guidelines instruct government employes to treat them like any other official of their status.

In the other -- when acting as private citizens paying taxes, performing military service or collecting Social Security, for example -- they are "to "seek no special favor, nor are they to be granted any."