Examined by Deputy District Attorney Marcia Clark, Mr. Martz distanced himself still further from Dr. Rieders. He offered a bar graph comparing the number of ions associated with EDTA that were found in samples of the Simpsons' blood taken from the test tubes to those found in blood samples from the gate and sock. Those taken from the test tubes resembled orange skyscrapers; those from the gate and sock looked like empty city blocks.

Mr. Martz also restated that tests of his own blood showed the possible presence of EDTA, a preservative used in breakfast cereal, mayonnaise and other foods, in the same trace amounts as on the gate and sock. But what looks like EDTA, he said, could just as easily be a number of similar chemical compounds, or contamination from the testing instrument.

"It's only logical to assume that if a person is eating EDTA, some of it will be in their blood," he said. "The question is how much. I don't think anyone knows today."

But Mr. Blasier got Mr. Martz to concede, for instance, that he had never been asked previously to look for EDTA in blood. Mr. Blasier even pointed out that there was nothing special about special agents, since every F.B.I. agent is one.

Mr. Blasier has depicted Mr. Martz as a hired gun, noting that when the prosecution enlisted him last February, it was, as Deputy District Attorney Rockne Harmon put it in a letter, "to refute the possibility" that the sock contained EDTA.

In rapid-fire questions Ms. Clark asked him: "Sir, did you take that to mean that we were demanding a particular result from you? Would it have mattered to you if you thought we had been? Did you take that to mean anything more than the confidence that the prosecution felt that they were not planted?"

To each question, Mr. Martz said, "No."

Testimony throughout the day was arcane, filling the air with terms like "full daughter spectrum" and "negative ion mode." At one point, Mr. Martz said, "Some of the molecular passes through the quadripole -- in this case, the quasi-molecular ion."