Transcripts from mitochondrial (maxicircle) genes of kinetoplastid organisms undergo RNA editing characterized by a series of reactions that insert and delete uridine nucleotides within the sequence of the pre-mRNAs. Guide RNAs, which complement fully edited mRNAs, provide the information for the edited sequence by an unknown mechanism. We report here that guide RNA molecules associate with other mitochondrial components to form four specific, stable ribonucleoprotein complexes. The complexes form very rapidly at a low monovalent cation concentration, and their formation is blocked by heparin or pretreatment of the mitochondrial lysate with SDS. ATP hydrolysis is not required but slightly stimulates complex association up to concentrations of 5 mM. The results are suggestive of a sequential assembly of the ribonucleoprotein complexes, and their possible involvement during the kinetoplastid RNA editing is discussed.