WASHINGTON — The Defense Department said Wednesday that it had mistakenly sent suspected live samples of anthrax to at least 51 laboratories in 17 states and three foreign countries, a higher number than officials had disclosed last week.

Pentagon officials said there were no indications that the mistakes posed a danger to the public because the samples were transported in low concentrations. There were also no signs that workers in the labs had become infected. But 31 workers were being monitored and given prophylactic treatment in case they were exposed, officials said.

Defense Department officials warned that the tally of labs that had received the shipments would probably grow.

The announcement Wednesday by the deputy defense secretary, Robert O. Work, was the third time that the Pentagon had increased its estimate of the number of anthrax samples that it said were inadvertently sent out. Mr. Work told reporters that mistakes at Dugway Proving Ground, the originating Army laboratory in Utah, included a failure to kill anthrax spores through radiation and a failure to confirm that spores had been killed before they were shipped out.