Lehman Brothers took unusual steps yesterday to reassure nervous investors about its exposure to turmoil in the subprime mortgage market, saying total activity from the sector accounted for an average of less than 3 per cent of revenues over the latest sixquarters.

The precise disclosure, which came on a conference call asLehman reported record first-quarter results, temporarily slowed a rapid decline in the bank's shares. The stock was down as much as 5 per cent in early trading yesterday after dropping 6 per cent on Tuesday on subprime fears.