WASHINGTON (AP) _ One of two men accused by Florida officials of looting the insurance company they ran denied any wrongdoing Thursday. The other refused to testify before Senate investigators.

Mark C. Sanford, president and chairman of Transmark USA, Inc., the parent of Guarantee Security Life Insurance Co. of Jacksonville, Fla., asserted his constitutional right against giving testimony that might be incriminating.

William Blackburn, former president of Guarantee, testified at length before the Senate Permanent Investigations Subcommittee and said he has cooperated fully with Florida authorities who are now running the company.

''I have been the subject of false accusations of fraud, breach of fiduciary duty and even criminal acts,'' Blackburn said. ''Many of the accusations have come from the Florida Department of Insurance and are absolutely baseless and without foundation.''

Florida Insurance Commissioner Tom Gallagher told the subcommittee Wednesday that Sanford and Blackburn systematically looted the company after luring policy buyers with promises of high earnings without disclosing Guarantee was heavily invested in high-risk ''junk'' bonds.

Transmark, controlled by Sanford and Blackburn, acquired Guarantee in 1984, when it had assets of about $90 million. It eventually grew to $1 billion in assets with 16,000 agents and policyholders in 41 states but became insolvent after the bottom dropped out of the junk-bond market in 1989.

Guarantee was placed in receivership last August. Although death benefits and annuities are being paid, policyholders are not being allowed to cash in their policies.

Questioned by Sen. Sam Nunn, D-Ga., the subcommittee, chairman, Blackburn quickly agreed that neither he nor Sanford - both former stock brokers - was qualified to run an insurance company. But Blackburn said he felt he was justified in taking $17.3 million out of Guarantee from 1984 through 1988, when he resigned.

Investigators said Sanford paid himself $37 million during the same period.

Florida officials say Guarantee attempted to make its financial position look stronger, avoided a special state tax and reduced its reserve requirements by engaging in several yearend ''phantom trades'' with Merrill Lynch & Co.

Guarantee would sign over much of its junk-bond holdings to Merrill Lynch in exchange for low-risk government securities around Dec. 31 and reverse the swap a couple days later.

Merrill Lynch officials described those transactions as totally above board and said they were reported regularly to Florida regulators.