Alternative splicing of the adenovirus-2 E1A pre-mRNA involves the use of three 5' splice sites and is modulated during infection because the 13S mRNA and 9S mRNA reactions are predominant during the early and late periods, respectively. We had previously reproduced in vitro the 13S to 9S modulation with nuclear extracts isolated from infected HeLa cells and shown that high molecular weight viral RNAs are involved in this modulation, most likely by sequestering or titrating general splicing factors. To further test this hypothesis, we titrated splicing factors from an uninfected nuclear extract using competitor RNA or by progressive inactivation of splicing factors with monoclonal antibodies. We found that the 13S to 9S modulation occurs when titrating only with certain RNAs (essentially adenoviral RNAs), and also by progressively inactivating the 9G8 SR splicing factor. The demonstration that late nuclear extracts contain levels of active SR splicing factors limiting for the 13S reaction has been made by complementation experiments. We show that late nuclear extracts do not complement SR factor-deficient extracts, whereas late extracts treated with micrococcal nuclease complement them. Furthermore, complementation of late nuclear extracts with each of the three 30-35-kDa SR factors (9G8, SC35, and SF2/ASF) restores an efficient 13S mRNA reaction. Thus, our results provide evidence that the 13S to 9S modulation is triggered through a titration of SR factors required for the 13S mRNA reaction by major late transcripts that accumulate in nuclei late in infection.