CHICAGO -- In the aftermath of last week's pioneering human gene therapy experiment to give a 4-year-old child a new immune system, skeptical scientists are taking the unusual step of going public with their doubts that it will work.

Speaking in Chicago at an international medical conference Wednesday, Arthur Bank, a physician and geneticist at Columbia University, charged that gene therapy researchers at the National Institutes of Health in Bethesda were driven by ambition and "not by good science."

Privately, many gene-splicers had been accusing the NIH team -- W. French Anderson and R. Michael Blaese -- of plunging ahead in a dangerous human experiment that had little scientific justification.

"The main impetus {for the experiment} is the need for French Anderson to be first to do gene therapy in man," Bank told 250 scientists and physicians attending the International Conference on Pre-Implantation Genetics. "This may turn out to be bad news for all of us."

Anderson, who has been researching gene therapy for 20 years, defended his experiment. "We survived every single regulatory hurdle they placed before us," he said, adding that he considers Bank a competitor. "The experiment is underway. Now we have to see if it works."

Last Friday, within hours after receiving the go-ahead, the NIH doctors began infusing genetically altered white blood cells into a vein of a Cleveland girl who suffers from a deadly disorder called severe combined immune deficiency.

The child was born without the gene that guides production of an enzyme needed to keep immune cells, known as T lymphocytes or T cells, alive and functioning.

By administering regular infusions of gene-altered T cells over the next 18 months, NIH researchers hope to provide the child with a working immune system. The girl completed her first infusion with no side effects, according to Anderson and Blaese, and went home a few days later.

The experiment is designed to determine whether the body will accept genetically engineered cells, allow them to produce their proper proteins and replicate themselves. If gene therapy works, it soon may be applied to major diseases.

Bank said the experiment was potentially dangerous because so little was known about what happens with altered cells that continue to divide.

Anderson's proposal to do the experiment was approved last July by the NIH's Recombinant DNA Advisory Committee by a vote of 16 to 1. The sole dissenter was Richard Mulligan, the Massachusetts Institute of Technology biologist who invented the technique Anderson used to deliver new genes into the patient's cells. He was harshly critical of the experiment. "They're misreading their own data," Mulligan said. "The possible benefits of the experiment don't outweigh the risks. It should be scuttled."