Nigeria's anti-corruption boss Ibrahim Lamorde has denied allegations that $5bn (£3.2bn) has gone missing at the commission he heads.
A Senate committee is investigating charges that assets and cash recovered by the Economic and Financial Crimes Commission (EFCC) were diverted.
Mr Lamorde told the BBC it was a smear campaign, as the EFCC was prosecuting the man making the allegations.
This showed that the case "should not be taken seriously".
Senator Peter Nwaoboshi, who is championing the investigation, was passed the petition by George Uboh, a security consultant.
Mr Uboh is currently fighting charges of stealing government property, which he denies.
Mr Lamorde said that even if all the money recovered by the EFCC was added up, together with its funding from government, it would not total $5bn.
"How can we divert an amount we don't have?" he asked.