The three-diiiiensional configuration and
cheiriicol composition of leptoniei-es in the inouse
\ e i i t r i c ~ i l a r cardiac inuscle fibre were electron
iiiicroscopically und iminuiiocytochemically in\-estigated
~ising seinithin sections and specific antibodies agairist
actin. the intei-mediate filament proteins desinin and
viiiientin. anci the aciin-binding proteins a-actiriin,
filainin and vinculin. The leptomeres appeared columnar
in shape arid periodically segmeiited by electron-dense
tiisc-like septa. The electrori-lucent areas betweeri these
septa uere composed of fiiie interlinked filainents
running obliquely to the major axis cif the Ieptomere.
Actiii was localized in the electron-dense lii-ies of the
leptoineres but not in the fine filaments. No reaction
was. however. detected for desinin. vinientin. \zi~iculin.
rilainin and a-actinin. The preserit results suggest.
therefore. that the leptomeres inay not have a contractile
functio