Antibodies to calf thymus histone H2B were purified by chromatography on DEAE-cellulose and injected into oocyte nuclei of Pleurodeles waltlii. As shown by indirect immunofluorescence these antibodies cross-reacted strongly with corresponding histones associated with lampbrush chromosomes. Shortly after injection the lateral loops of the chromosomes retracted into the chromomeres and by 3 h postinjection the 'lampbrush' appearance was completely lost and the chromosomes appeared in light-microscopic preparations as rod-like structures consisting of longitudinally coalesced chromomeres. In control oocytes injected with non-immune immunoglobulins or antibodies against a ubiquitous transcript-associated protein no morphological alterations of the lampbrush chromosomes could be observed. Electron microscopic spreads of chromosomes prepared at various times after injection of anti-H2B revealed a progressive loss of transcriptional complexes from the loop axes. Finally, higher-order chromatin configurations, like supranucleosomal globules ('superbeads') or cable-like chromatin strands 50-60 nm thick predominated, indicating complete transcriptional inactivation of all chromosomal regions. The results indicate that H2B antibodies react specifically with histones associated with the transcribed DNA of lateral loops in their native state. The resulting antigen-antibody complexes seem to inhibit progression of the RNA polymerases along the template, thus causing the premature release of transcripts, a process analogous to the stripping effect of actinomycin D. The demonstration of histones associated with heavily transcribed regions, which are not compacted into nucleosomes but largely extended, supports the current concept that unfolding of nucleosomes to allow transcription of the DNA does not involve dissociation of histones. In contrast, amplified ribosomal RNA genes are unaffected by injected H2B antibodies. This does not necessarily indicate absence of histones from nucleolar chromatin, since we do not know whether it is accessible in vivo to antibodies or whether the histone antigenic determinants are masked by the presence of other proteins. The technique of injecting specific antibodies should be widely applicable when analysing the in vivo distribution of chromosomal components at the electron-microscopic level and when studying complex metabolic processes, like the cleavage and modification of RNA, by selective inhibition of defined enzymic steps.