Six high molecular weight, rapidly labelled RNA species were detected in freely suspended callus cells of Petroselinum sativum by means of isotope labelling and electrophoretic separation in agarose-polyacrylamide gels. On the basis of their migration in the latter the RNA species were calculated to have the following molecular weights: 2.9×106, 2,4×106, 1.9×106, 1.4×106, 1.0×106 and 0.75×106 daltons. Thus they can clearly be distinguished from the two ribosomal RNA species (1.3×106 and 0.7×106 daltons). During incubation of the cells with [3H]methyl-methionine as a methyl donator all six components incorporated radioactivity rapidly. With [3H]nucleosides or [3H]orotic acid as precursors the 2.9×106 and the 2.4×106 daltons RNA were labelled within 10 min, while the other high molecular weight species appeared after about 20 min of labelling.