The labeling of m7GpppN1mpN2p caps with L-[methyl-3H]methionine on short (100-500 nucleotides) heterogeneous nuclear RNA (hnRNA) chains of HeLa cells is increased 2-3 times, but the labeling of caps on longer (> 2000 nucleotides) hnRNA chains is decreased by .apprx. 80% by treatment of the HeLa cells with 5,6-dichloro-1-.beta.-D-ribofuranosylbenzimidazole (DRB). The experimental conditions were as follows: HeLa cells were treated with 75 .mu.M DRB for 40 min before labeling and also during the 30-min pulse of L-[methyl-3H]methionine; actinomycin D (0.05 .mu.g/ml) was used to suppress ribosomal RNA synthesis. Control cells received no DRB. The RNA was separated in Me2SO gradients to ensure no aggregation. Labeling of cells with [3H]uridine for 10 min and separation of RNA by these techniques reconfirmed the findings, that 70-80% of the synthesis of hnRNA (> 1000 nucleotides) is sensitive to inhibition by DRB, but that 20-30% is resistant. This analysis of the methyl-labeled caps provides evidence that DRB causes early termination of a large fraction (.apprx. 70-80%) of hnRNA precursor chains. In contrast to the finding of continued synthesis and accumulation of short m7GpppN1mpN2p-capped chains in the presence of DRB, the synthesis of m2,2,7GpppN1mpN2mp-capped small nuclear RNAs was inhibited by .apprx. 70% by DRB.