In '76 Libya targeted US envoy

One reason US officials are inclined to take seriously intelligence reports of a Libyan assassination plot against President Reagan has been the Eilts case of 1976-77.

Although there is incomplete proof of a plot against Mr. Reagan, US officials say there was irrefutable evidence of a Libyan plot against the then US ambassador to Egypt, Hermann F. Eilts.

In a telephone interview, Ambassador Eilts, now with Boston University, described that evidence as ''absolutely firm.'' He revealed that President Carter decided to issue a private warning to Qaddafi and then presented him with evidence, thus bringing the plot to an end.

''The pattern was exactly the same in both cases - hit teams are discovered and then come the Libyan denials, with a demand that we produce the evidence,'' said Ambassador Eilts.

The Venezuelan terrorist Carlos, thought to be working for Qaddafi, was organizing the plot against him, according to Eilts. But one member of a two-man ''hit team'' of Egyptian expatriates was captured, he said, thus disrupting the plot.

Eilts thinks that by going public on the issue, President Reagan may succeed in deterring Qaddafi, but he adds, ''With Qaddafi, you can never be sure.''