Mammalian ELAV proteins bind to polyadenylated messenger RNAs and have specificity for AU-rich sequences. Preferred binding sites in vitro include the AUUUA pentamer and related sequences present in the 3' untranslated regions of many growth regulatory mRNAs. Human ELAV (hELAV) proteins have been implicated in post-transcriptional regulation of gene expression by their effects on the stability and translatability of growth regulatory mRNAs. We have examined the intracellular localization of ELAV proteins in neurons and in tumor cells of neuronal origin using indirect immunofluorescence, confocal microscopy and biochemical separation. Mammalian neuronal ELAV proteins are found predominantly in the cytoplasm of cells in mRNP complexes termed alpha complexes which, when associated with polysomes, form large and high density ss complexes, as assayed by glycerol and accudenz gradients, respectively. Puromycin, cytochalasin or EDTA treatments disrupt beta complexes causing the release of alpha complexes, which then appear, by confocal microscopy, as large hELAV mRNP granules associated with microtubules. Association of partially purified hELAV mRNP alpha complexes with microtubules was confirmed by in vitro reconstitution assays. Furthermore, colchicine treatment of cells suggested that association of hELAV mRNP alpha complexes with microtubules is also necessary for the formation of ss complexes. Our data suggest a model in which a subset of mRNAs is associated with microtubules as ELAV mRNP particles (alpha complexes) which, in turn, associate with polysomes to form a translational apparatus (beta complex) that is, through polysomes, associated with the microfilament cytoskeletal network. hELAV proteins in these mRNP granules may affect post-transcriptional regulation of gene expression via the intracellular transport, localization and/or translation of growth regulatory mRNAs.