An as-yet unexplained biological difference between blacks and whites may account for the bact that blacks are twice as likely to deliver babies with low birth weights.

A sweeping study of almost 30,000 births found that even when other factors were considered -- including smoking and alcohol use, level of education and age of the mother -- blacks were still much more likely to have low-birth-weight infants.

Those tiny infants, weighing less than 5 1/2 pounds, face a higher risk of death or severe health problems in their first few months.

The study was undertaken at Kaiser-Permenente health clinics in northern California and reported in the current Journal of the American Medical Association.

''The conclusion of the analysis is stunning in its simplicity,'' Dr. Wendy Baldwin of the National Institute of Child Health and Human Development wrote in an editorial accompanying the study. ''Despite controls for likely pathways of influence on birth weight, and despite the bias of excluding the poorest segment of society (and the richest), the disparity between blacks and white endures.''

Other factors, including low income, make low-birth-weight delivery more likely, Baldwin said, stressing the importance of good prenatal care.

In the Kaiser study, headed by Patricia H. Shiono of the National Institutes of Health, 3.6 precent of the white infants were low-birth-weight, compared with 7.7 percent of the black infants. In both groups, national averages were higher: 5.6 percent for whites, 12.5 percent for blacks.

In her editorial, Baldwin said ''biological or genetic differences'' may be the key to understanding the disparity. ''However, our understanding of racial differentials is clearly in need of improvement."