Processed intermediates of aldolase B mRNA precursor in nuclei were analysed by hybridization with cDNA or intron-specific DNAs. The results indicated that there is, in part, a preferred order in the removal of eight introns from the primary transcript. The largest intron (IVS 1) may be selectively eliminated at an earlier step during the processing, while others (IVS 2 and IVS 7) are not. In addition, the data suggest that the removal of four to five introns from the precursor occurs rapidly after transcription, and that the resulting partially processed molecules persist rather stably before being completely processed.