Translational regulation of protein synthesis accompanies sclerotization in Physarum polycephalum. Plasmodial and sclerotial poly(A)+ RNA were translated in a message-dependent wheat germ lysate in the presence of the cap analogue 7-methylguanosine-triphosphate to determine whether 5' structural alterations in mRNA accompany translational repression. The translation of plasmodial and sclerotial poly(A)+ RNA was reduced to identical levels suggesting that both RNA populations are capped. The 5'-termini of plasmodial and sclerotial poly(A)+ RNA were identified as m7G5'ppp5'Cm. Alterations in the 5'-cap of mRNA during sclerotization do not appear to be responsible for translational dormancy.