NEW YORK — Morgan Stanley, which had the largest trading-revenue drop among major US banks last quarter, lost money in that business on 15 days in the period, up from eight days a year earlier.

The firm’s traders generated more than $100 million on three days in the period, compared with seven days in the second quarter of 2011, the company said Monday in a filing. None of the daily losses exceeded the firm’s value-at-risk, a measure of how much the bank estimates it could lose on 95 percent of days.

Morgan Stanley had a 48 percent year-over-year decrease in trading revenue, excluding accounting gains, led by a 60 percent drop in fixed-income revenue. Chief financial officer Ruth Porat said last month that some clients shied away from doing business with the bank as they waited to see whether Moody’s Investors Service would cut the firm’s credit rating two or three grades. Moody’s lowered the assessment two levels in June.