The transformation is complete. Bianca Jagger, show-biz personality, wife (then former wife) of megastar Mick Jagger, is no more.

At 38, she is Bianca the concerned citizen, Bianca the Nicaraguan patriot, Bianca the crusader against Reaganite arrogance in Central America, Bianca the hammer of the Contra rebels. She is Bianca, mother of vulnerable, teen-age Jade."Life is not necessarily more fun than before, but it is more interesting," she says.

She is an undergraduate again, learning filmmaking at New York University, going to classes from her apartment on Park Avenue.

In London for a television chat show on the Sandinistas, the serious side was well in evidence. With genuine pride, she recounts how, visiting Honduras with a U.S. congressional delegation, she saved 40 refugees from a grisly fate at the hands of marauding Salvadorean soldiers.

She is happy to have helped the British Red Cross in Central America.

She glows at the memory of taking relief supplies to the victims of the great Managua earthquake in 1972. The experience of seeing the homeless in her native city, she says, was the watershed in making her into a more thinking person.

The commitment is authentic, unfeigned. The fiery companion of Mick Jagger, who married her a few months before Jade was born, is now the activist. Undogmatic, skeptical, no unquestioning supporter of the Sandinistas, but an activist nevertheless.

"The real question is, `Do the Nicaraguans have the right to decide for themselves or not?' " she says. "They elected this government, and the government is not perfect. They do many things that I regret that they do. But the fact is that we are not a colony. It is not the place of the United States to say, `We want them to have this kind of government or that kind of government."'

Bianca has made herself a successful post-Mick Jagger life. "In Nicaragua I was brought up in a conservative, very traditional atmosphere where you always thought in terms of a woman and a man. Now I'm able to enjoy things on my own, for myself, as a sort of wandering Nicaraguan," she says.

But she adds quickly, "Of course, I would like to be able to share them."

In black leotard and white blouse, drinking camomile tea and picking at a fruit salad, she is still wary and careful in front of the camera lens. She is jealous of her image.

The political concern is genuine. But the Bianca of the old days has not gone away. Matured, perhaps, rather than transformed.