microlab,
They are not interchangeable. The 74LVC244 is a low voltage CMOS device with a power supply range of 1.2 to 3.6V. The 74LS244 is a low power Schottky device with a power supply range of 4.75 to 5.25 Volts. The inputs and outputs of the 74LVC244 are, however, compatible with low power Schottky logic levels. Search Google for data sheets of both parts.
Regards,
Kral