This study investigates the interaction between antibodies and psoralen-modified RNA, aiming to develop probes for studying RNA structure. The researchers immunized rabbits with 4'-aminomethyl-trioxsalen (AMT)-modified poly(A,U) complexed with methylated bovine serum albumin (mBSA). The resulting antisera were characterized using competition radioimmunoassays (RIA) and enzyme-linked immunosorbent assays (ELISA). AMT-poly(A,U) was found to be over 10,000-fold more reactive than unmodified poly(A,U) or AMT alone. The antisera cross-reacted with varying degrees of AMT-modified RNA and DNA, with AMT-uridine typically ensuring strong reactivity. The amino group of AMT contributed minimally to antibody binding, and binding was not significantly affected by high ionic strength, suggesting that ion pair formation is not involved. Murine encephalomyocarditis virus (EMCV) replicative intermediates and cellular RNA and DNA were modified by psoralen in intact cells, indicating that these molecules possess detectable stretches of base pairs. The described antibodies will be useful for studying the secondary and tertiary structure of RNA both in vitro and in intact cells.This study investigates the interaction between antibodies and psoralen-modified RNA, aiming to develop probes for studying RNA structure. The researchers immunized rabbits with 4'-aminomethyl-trioxsalen (AMT)-modified poly(A,U) complexed with methylated bovine serum albumin (mBSA). The resulting antisera were characterized using competition radioimmunoassays (RIA) and enzyme-linked immunosorbent assays (ELISA). AMT-poly(A,U) was found to be over 10,000-fold more reactive than unmodified poly(A,U) or AMT alone. The antisera cross-reacted with varying degrees of AMT-modified RNA and DNA, with AMT-uridine typically ensuring strong reactivity. The amino group of AMT contributed minimally to antibody binding, and binding was not significantly affected by high ionic strength, suggesting that ion pair formation is not involved. Murine encephalomyocarditis virus (EMCV) replicative intermediates and cellular RNA and DNA were modified by psoralen in intact cells, indicating that these molecules possess detectable stretches of base pairs. The described antibodies will be useful for studying the secondary and tertiary structure of RNA both in vitro and in intact cells.