In the world of "dirty money," public attention has focused on the need for better policing of big banks. Now congressional investigators are looking at another villain -- the Internal Revenue Service.

The Treasury Department has fined six banks this year for failure to fulfill reporting requirements that could tip authorities to large-scale money laundering linked to drug trafficking and other crimes.

But House investigators now say one of Treasury's own -- the IRS -- has left open a convenient secondary money-laundering route and has failed to use its legal authority to develop criminal investigations in the area.

Under the Bank Secrecy Act, the IRS is responsible for assuring that thousands of "secondary financial institutions" -- such as check-cashing facilities, finance companies and precious metal or coin dealers -- report all transactions of more than $10,000.

But the agency "continues to devote grossly insufficient resources" to the task "and its efforts lack coordination and consistency," Rep. Doug Barnard Jr. (D-Ga.), a Government Operations subcommittee chairman, wrote to IRS Commissioner Roscoe L. Egger Jr.

"At the current level of resources and commitment, the currency transaction reporting program cannot succeed in meeting its intended purposes of detecting criminal activity and unreported income in the underground economy," Barnard said in an Oct. 25 letter.

Subcommittee investigators found that by December 1984, the IRS had identified only 3,014 of an estimated 15,000 businesses involved in such transactions.

They also found that the agency conducted only 644 compliance checks at such businesses in 1984, less than one-third the number made in 1981.

Accurate reports on large cash transactions can be useful in criminal investigations -- as demonstrated by a 1982 internal IRS audit obtained by the subcommittee, an investigator said.

In samplings of two IRS districts -- Cincinnati and Louisville -- where about 5,000 currency transaction reports were filed from 1976 to 1980, the report found 42 persons involved in three or more transactions. Of these, 10 individuals involved in 57 transactions totaling $1.3 million had no record of ever filing tax returns.

One individual who had filed a tax return in 1980 had 27 currency transactions in 1980 totaling $1.2 million, yet had reported an adjusted gross income of only $2,295. This person also had a number of large transactions in 1978 and reported a small income then. Four others with large transactions reported income of less than $10,000 on their 1980 tax returns.

Five in the group had criminal records and two were under investigation for narcotics violations. But officials had made no use or minimal use of the transaction reports to develop criminal investigations, auditors found.

The IRS declined immediate comment but said Egger will reply directly to Barnard.