The Montgomery County police chief and state's attorney announced yesterday that they would not prosecute the mother of an abandoned infant for child abuse if she will turn herself in to social service authorities.

"We're not interested in prosecuting her or persecuting her," said Police Chief Robert J. de Grazia after a conversation with State's Attorney Andrew Sonner and Harriet Herrman, social services director. "We simply want her to answer some questions so we can find the best home for the child."

A 10-day-old baby wrapped in a blue blanket and nursing a bottle of warm milk was abandoned on the steps of a Catholic church in Wheaton on Monday with a note asking that "good care" be taken of "David James."

"We want the mother to know we'd like to help her," said Hermann. "Most parents want their children to have a good life, but sometimes they don't know how to do that. Obviously the mother cares for her baby. The bottles with the child were warm and there were diapers there."

If the mother does not wish to be reunited with the infant, social service counselors want to talk with her in an effort to find the best adoptive home.

The child, with dark hair and blue eyes, was examined at Holy Cross Hospital and placed temporarily in a foster home. "He's very healthy," said Hermann.

Only one other child has been found abandoned in Montgomery County in recent years. In such cases, it is appropriate that the law "locks the other way" said County Executive James P. Gleason.