Human-avian influenza reassortants possessing the HA gene of the avian parent virus were tested for their ability to replicate in MDCK cells at 37 degrees C and 31 degrees C. Both avian parent viruses, A/Duck/Ukraine/1/63 (H3N8) and A/Duck/Hoshimin/014/78 (H5N3) induced an efficient multi-cycle infection at 37 degrees C, but replicated poorly at 31 degrees C, whereas the human parent virus, MDCK-adapted variant of A/USSR/90/77 (H1N1) strain, replicated efficiently at both temperatures. The reassortant clone possessing the HA gene of A/Duck/Ukraine/1/63 virus and the other 7 genes of A/USSR/90/77 virus replicated at both temperatures almost as efficiently as the human parent virus. Among the reassortants between A/Duck/Hoshimin/014/78 and A/USSR/90/77, the clones possessing the HA and NA genes of the avian strain, or the HA, NA, NP, and NS genes of the avian strain, and the other genes of the human parent virus, replicated poorly at both temperatures, especially at 31 degrees C, whereas the reassortant possessing the HA, NA, and M genes of the avian virus replicated at both temperatures fairly efficiently. The results are discussed in connection with the limitations imposed by different genes upon avian influenza viruses' ability to replicate in mammalian cells.