2-5A[ppp(A2'p)nA] and related materials accumulate to greater than micromolar concentrations in vaccinia virus-infected HeLa cells and in interferon-treated, vaccinia-infected HeLa, CV1, and L929 cells even when virus replication is not inhibited. A variably complex mixture of authentic 2-5A (n = 2 to 4), nonphosphorylated cores [(A2'p)nA; n = 2 to 5], and additional compounds of unknown structure was observed.