THE PROBLEM with being in a line, think the characters in Israel Horovitz's Line , is getting a good place and holding onto it. Under Ken Bartels' direction last weekend at the Loeb Experimental Theatre, the cast carefully minded its p's and queues.

In the course of the one-act play, five people converge on THE LINE, a six-inch strip of tape, and vie to be first. It doesn't matter why they have come. Fleming wants to buy baseball tickets. Arnall thinks he is going to see a movie. But only Steve, excellently played by Richard A. Green, knows that there isn't really a reason.

Fleming, a paunchy, coarse oaf played by Stephen Porter with obvious enjoyment, has camped by the line all night to make sure he beats everybody else. But it is Steve who always controls the situation. Not until he pads quietly into the room is the line defined. He slithers up to Fleming, stares wildly into his eyes, and informs him. "There's never been a real first place." Yet he stirs up the competition that follows as three others enter and try out their techniques for moving up in line.

For Molly, the only woman in the lot, the means is of course-sex. Chris Dekker comes across in the role as suitably brassy but a bit weak. She reaches her best moment when she gloats over her temporary position at the head of the line.

Guy Rochman is thoroughly convincing as Dolan, a slick con artist with oily black hair and a classy neon green suit. He propounds the "Underdog Philosophy." Hanging back, waiting for a chance to sneak ahead, he files his nails and keeps his cool.

Molly's mousy husband, Arnall, well played by Jan Madsen, is more concerned about germs than his place in line, but he to learns to be competitive. He decides that, like the other three men, he wants a piece of the action, with which Molly is so liberal, even though sex makes him sick. And in the end he too thinks "First is good."

Though the struggle to be first is just a game to Steve, he is dangerously serious. He demonstrates his power by twice changing the line's direction. He can even dance out of his place without losing it, by distracting and terrifying the rest with music.

In an almost demonic frenzy, when all is in commotion, he rips up the tape and swallows it, expecting and hoping that someone must then kill him. His disappointment at continuing to live is severe. After he coughs the tape up in five bits, the other four characters each seize a piece, triumphantly placing themselves first in their own private lines. He alone sees that their victory is meaningless and, with a laugh, throws his piece down.

Recent successes in the Ex prove its suitability as an outler for the creative impulses down on Brattle St. Two weeks ago Liz Coe's production of Joe Egg was favorably received and Pope Brock's starring performance roundly applauded. Last weekend an Ex staging of Sartre's No Exit offered a useful counterpoint to the mainstage presentation of Dirty Hands . Ex tickets available free at the box office the day before each show, are going so fast now that you should try to be first in line yourself.