Transplanting a vein from a leg is one of the critical steps in coronary-bypass operations, but thousands of people don't have a healthy vein available. Now two young companies are close to offering the solution, in two radically different forms. One is biologically compatible plastics and the other is treated animal veins -- providing new options for patients who suffer from circulatory diseases.

Donald J. Lyman, president of Vascular International Inc., spent a decade designing and testing materials for use in small-diameter blood vessels. By manipulating the molecules of various polymers, Lyman eventually produced a vessel one millimeter in diameter, about half the width of the lead in a No. 2 pencil. More importantly, Lyman's plastic vein appears to solve a longstanding problem: the tendency of artificial vessels to clog. Last year, Lyman licensed the technology from the University of Utah, where he is a bioengineering professor, for $148,000. He then started his company, raising $329,000 from stock offerings.

Vascular International is beginning human tests of the blood vessel -- but not soon enough for Lyman. "One of my friends at Stanford [University] hoped to be one of our first guinea pigs," he says. "But he didn't live long enough."

Xenotech Laboratories Inc. takes a quite different approach. "It turns out that nature is the best designer," says Charles Martin, chairman of Xenotech Laboratories, of Irvine, Calif. The company treats animal vessels with chemicals that preserve the tissue and thus inhibit the body from biologically rejecting it. The four-year-old company anticipates beginning human tests within the next year, and Martin says animal studies indicate the vessels will work at diameters as small as four millimeters. Further along in development are ligaments, tendons, and heart valves from animal tissue -- suggesting that the future bionic person may be part human, part pig.