A Florida sandhill crane died yesterday at the Patuxent Wildlife Research Center near Laurel, bringing to 15 the number of cranes that have died of an undetermined ailment since mid-September.

The mysterious condition, which has killed three of the wildlife center's endangered whooping cranes, continues to puzzle scientists but also seems to have abated, a spokesman said yesterday.

Matthew Perry, an information biologist at the U.S. Fish and Wildlife Service's Patuxent facility, said scientists are investigating the possibility that the birds ingested a natural toxic agent, perhaps from fungi that grow in their pens, on the grass and the soil. Perry said common avian diseases have been ruled out and that scientists are awaiting results of laboratory tests that could point to toxins, which are difficult to detect.

About 100 of the center's 285 cranes are still afflicted by the ailment, but Perry described their condition optimistically, saying they are "weakened, but only three are in real serious condition. This is a great improvement from what we had in the past." He said that many of the ailing birds that earlier had required tube-feeding by veterinarians were eating on their own.

The condition, which was detected Sept. 17, two days before the first crane died, manifests itself in diminished appetite, Perry said. The birds stopped eating, became listless and required force-feeding. Post-mortems have revealed a variety of problems, including kidney dysfunction and inflamed intestines. But besides being emaciated, the dead cranes have shown no common pathological agent.

"We really don't have a good, firm idea now about what's caused this problem," Perry said. "We haven't come too close to figuring it out . . . Some of the best minds are working on this and we still haven't pinned it down."