Research on the informal support networks of older persons recognizes that network size and composition (i.e., family vs. nonkin) may have important consequences for care. Factors that determine these aspects of networks among older blacks, however, have not been explored systematically. The present study examined the relationship of a group of sociodemographic, health, family, and availability factors to the size and composition of the informal support network. The data were taken from the National Survey of Black Americans and constitute a nationally representative sample (N = 581) of older blacks (55 years and older). The results for several of the sociodemographic factors (i.e., sex and marital status) are consistent with previous work. Regional differences in network dimensions, however, suggest new areas of inquiry. The findings underscore the importance of availability and family factors in support relationships and the relative ineffectiveness of health factors as predictors of network size and composition.