The role of contractile proteins in the structural organisation of the interphase nucleus and of metaphase chromosomes is largely unknown. Actin has been found in interphase nuclei of different species, especially in association with condensed chromatin. In the germinal vesicle (nucleus) of Xenopus oocytes, actin has been localised in the nuclear gel supporting the chromosomes and the extrachromosomal nucleoli. It has been reported that the premeiotic lampbrush chromosomes in these germinal vesicles are positively stained for actin and tubulin by the immunoperoxidase technique. Moreover, the longitudinal contraction of these chromosomes is ATP dependent. Therefore it has been suggested that actin participates in the structural organisation of the highly specialised lampbrush chromosomes. However, actin is not a major component of the metaphase chromosome scaffold. The results reported here suggest that actin is involved in the condensation of Xenopus chromosomes.