In an attempt to understand the genetic interactions between nuclear and mitochondrial genomes leading to mitochondrial biogenesis, different combinations of known nuclear and mitochondrial mutations have been constructed by microinjection. Eleven different tetrazolium resistant mutant strains, many clearly affecting mitochondrial function, were injected with mitochondria from four different erythromycin resistant mitochondrial mutants. Cases were found in which mutant mitochondria were unable to replicate in tetrazolium resistant mutants. The successful mitochondrial transfers were characterized for growth rate, temperature and cold sensitivity. Several selected combinations were characterised also for cytochrome spectra and cyanide resistance. Many different phenotypes were produced by the interaction of the different nuclear and mitochondrial mutations. These ranged from a positive interaction in which mutant mitochondria were selected by a nuclear mutant in preference to wild-type, through apparent absence of interaction, to negative interaction in which the mitochondrial-nuclear combination was temperature sensitive even though both 'parents' were thermoresistant. The possible molecular basis of these interactions is discussed.