State Attorney General Jim Mattox was acquitted tonight of a commercial bribery charge stemming from allegations that he threatened a Houston law firm's lucrative bond business.

Supporters of Mattox cheered the jury's verdict, which came after three hours of deliberations and five weeks of testimony.

After state District Court Judge Mace B. Thurman Jr. dismissed the jurors, Mattox joined them in the jury room to thank them "from the bottom of my heart."

In a brief statement, Mattox told the panelists, "I never made those statements they say I did. Had I done so, I would have walked over here and taken my medicine." Mattox faced up to 20 years in prison and a $5,000 fine if he had been convicted.

The former three-term congressman was indicted by a Travis County grand jury 18 months ago for allegedly threatening the bond business of the Houston law firm of Fulbright & Jaworski if one of its lawyers, Thomas McDade, persisted in trying to question his sister, Janice, in an oil royalties case. The attorney general's office must approve the issuance of all public bonds; law firms collect fees based on that approval.

In closing arguments, defense attorney Charles Burton called Mattox "a truthful man" who fell victim to McDade, who was representing Mobil Oil Corp. in the $1.7 billion oil royalties lawsuit.

The suit was filed by Texas rancher Clinton Manges, who claimed that Mobil for years had been illegally drilling on his land. The state, seeking millions of dollars in lost oil royalties, joined the suit on Manges' side. Mattox represented the state.

It was McDade's repeated efforts in 1983 to question Janice Mattox, a Dallas lawyer, that started the events leading to the attorney general's indictment.

Evidence in the trial, which began Feb. 11, included tape recordings of telephone calls between Mattox and the offices of Fulbright & Jaworski. Prosecutors alleged that in a June 17, 1983, conversation, Mattox threatened Wiley Caldwell, who heads the firm's bond section, that he would put an embargo on $160 million of the firm's pending bond business if McDade proceeded with plans to subpoena Janice Mattox in the oil rights case.

No tape of such a conversation was introduced at the trial, and Mattox denied making such a threat.

Representatives of 17 governmental bodies who were clients of the law firm testified that they had had no delays in getting state approval for the issuance of municipal bonds.

But Caldwell, a former Mattox fund-raiser, testified that the attorney general had "threatened war" in the phone call and said the law firm "would get approval of no bonds" until McDade dropped the subpoena for Janice Mattox. It was later dropped.

In a taped call to the firm on June 20, 1983, Mattox said he could no longer trust Fulbright & Jaworski and that he had instructed his staff to "make sure the I's are dotted and the T's are crossed" on its bonds.

Chief defense lawyer Roy Minton said Mattox was only doing his duty when he told the firm that its bonds would be checked closely. Minton also said it was proper to treat McDade "like a batch of snakes" because he was not trustworthy.

District Attorney Ronnie Earle said Mattox had confused his private interest with the public's business. "The man has a problem with vindictiveness," Earle said. "He has a problem with being a bully."