[AS PER AMENDMENT 9/24/97: This study is divided into sections, as follows: Part 1A is a single-dose study in neonates 0 to 48 hours of age. ABC dose escalations are made until a dose is identified that meets toxicity guidelines and demonstrates a minimal target area under the concentration curve (AUC) of 2,000 ng-hr/ml. Part 1B is a similar single-dose study in infants 3 to 7 days of age with escalation as per part 1A. Part 1C is an identical single-dose study in infants 21 to 28 days of age but starting at the dose identified in Part 1B. Part 2 is a multi-dose study to examine a 6-week dosing regimen for ABC and ZDV for infants 0 to 48 hours of age. The dosing regimen for ABC is defined in Part 1A for the first 48 hours of life, the dose defined in Part 1B for Days 3 through 20 of life, and the dose defined in Part 1C for Days 21 through 42 of life.] [AS PER AMENDMENT 7/29/98: Enrollment to Parts 1A and 1B will remain open; Part 1A will enroll a minimum of 4 patients as planned, and Part 1B will enroll 3 additional patients.]