  A new data format for Monte Carlo (MC) events, or any structural data,
including experimental data, is discussed. The format is designed to store data
in a compact binary form using variable-size integer encoding as implemented in
the Google's Protocol Buffers package. This approach is implemented in the
ProMC library which produces smaller file sizes for MC records compared to the
existing input-output libraries used in high-energy physics (HEP). Other
important features of the proposed format are a separation of abstract data
layouts from concrete programming implementations, self-description and random
access. Data stored in ProMC files can be written, read and manipulated in a
number of programming languages, such C++, JAVA, FORTRAN and PYTHON.
