Assets of the nation's 539 money market mutual funds rose by $2.85 billion in the latest week to a record $471 billion, the Investment Company Institute reports.

The rise in the seven-day period ended Wednesday followed a $61.2 million decline in fund assets the previous week to $468.15 billion. That decrease originally was reported as $101 million but the Washington-based mutual fund trade group later revised the figures.The Investment Company Institute said that in the latest week, assets of 260 general purpose funds rose $1.82 billion to $181.84 billion, assets of 90 broker-dealer funds increased $368.9 million to $155.85 billion, and assets of 189 institutional funds rose $659.1 million to $133.35 billion.

The seven-day average yield on money market mutual funds fell in the week ended Tuesday to 6.15 percent from 6.19 percent the previous week, said Donoghue's Money Fund Report, a trade journal based in Hol-liston, Mass.

The 30-day average yield fell to 6.26 percent from 6.34 percent, Don-oghue's said.

The average maturity of the portfolios held by money funds was 54 days, unchanged from a week earlier, Donoghue's said.

The newsletter Bank Rate Monitor said its survey of 100 leading commercial banks, savings and loan associations and savings banks in the nation's 10 largest markets showed the effective annual yield available on money market accounts fell to 5.59 percent as of Wednesday from 5.62 percent a week earlier.