We consider an augmentation problem on mixed graphs that generalizes and unifies two augmentation problems considered by \textit{K. P. Eswaran} and \textit{R. E. Tarjan} [ibid. 5, 653-665 (1976; Zbl 0346.05112)]. The mixed augmentation problem has applications in the design of communication networks, and forms of mixed augmentation problem are central in statistical data security problems discussed in [Yale Univ. Tech. Report, August 1984]. We solve the augmentation problem on mixed graphs in time linear in the number of edges of the graph, the same time bound obtained in [loc. cit.] for each of the two special cases.