Nineteen equations of state for hard-sphere fluids and six for Lennard-Jones fluids are reviewed from the literature, and used to calculate the isothermal compressibility. We show that the choice of which equation of state to use to calculate the isothermal compressibility of hard-spheres is most difficult for high densities. We also performed the hard-sphere calculation by using available numerical values of the radial distribution function, finding adequate results only at low densities. Moreover, two recently proposed analytical models that reproduce these radial distribution function values seem not to be valid for the calculation of the isothermal compressibility. For the Lennard-Jones system, we found first that the influence of attractive intermolecular forces is not negligible at any density; second, that the equations of state lead to different results for states near the phase transitions; third, that none of the equations of state considered here gave Values for the isothermal compressibility in full agreement with the available computer simulation results for states near the vapour-liquid coexistence curve; finally, that there was good agreement with available experimental values for liquid argon.