Maryland officials yesterday ruled out Pfiesteria piscicida as causing the skin rashes that appeared on the hands of two environmental investigators, whose symptoms helped trigger fears that the toxic microbe had returned to the Chesapeake Bay region.

Doctors were still evaluating a third person whose complaint of skin rashes and flulike symptoms early last week prompted authorities to caution people about the possible hazards of using Back Creek. The creek is on the Eastern Shore about 90 miles southeast of Washington.

Yesterday, Back Creek, which feeds the Manokin River in Somerset County, remained open to normal use. Although pfiesteria has been detected in some water samples, officials have found no fish with sores, said John Surrick, a spokesman for the Department of Natural Resources. Lesions on fish are an indication the normally benign microbe has changed into a dangerous toxin-spewing form.

The two investigators reported rashes on their hands late last week, said Robert Venezia, director of environmental health coordination for Maryland's Department of Health and Mental Hygiene.

"These two later cases just had simple skin irritations," Venezia said. "It's not pfiesteria-related illness. These guys are just fine."

Doctors do not know what caused the investigators' rashes, which could have been brought on by sun, wind, dampness, fungus or many other sources, Venezia said.

The finding sheds no light on what happened to the person who made the initial Back Creek report Aug. 9, Venezia said. He said doctors expect to finish evaluating that person in a few days.

That person was on the river for pleasure, but Venezia could not say whether it was for fishing, swimming or some other activity.

Venezia declined to identify the two investigators beyond saying they were among those who inspect rivers and take water and fish samples after reports of possible pfiesteria activity. River inspections are done by employees of Maryland's environmental and natural resources agencies, health officials, police and federal officials, he said.

Pfiesteria is believed to exist widely in a nontoxic, benign state in Chesapeake Bay and other estuaries. For reasons not well understood, it can change shape and behavior, attacking fish and emitting powerful toxins that damage nervous systems.

After 30,000 or more fish died amid pfiesteria blooms in the Eastern Shore's Pocomoke River in 1997, doctors concluded that the microbe had sickened 13 people in Maryland.

Since then, Maryland investigators have examined about 30 people who reported possible pfiesteria-related symptoms, but they ruled out the microbe in all those cases, Venezia said.

Those examined had reported symptoms such as skin rashes, headaches, chest irritation and muscle cramps, Venezia said.