The heating potentials of two newly-developed microwave interstitial antennas are reported in this paper. The longitudinal (parallel to the antenna) and transverse (over a plane perpendicular to the antenna) specific absorption rate (SAR) distributions of single and an array of four parallel antennas were measured in a muscle equivalent phantom and their performance characterized at 915 MHz in terms of the following parameters: peak depth (location of the profile peak with respect to the surface), 50% HL (effective heating length over which SAR greater than 50% of the peak normalized SAR), dead length (axial length at the antenna tip with SAR less than 50% of peak normalized SAR), and the variations of the specific absorption rate pattern relative to the depth of insertion. The results are analyzed and discussed in terms of these parameters and other factors important in the clinical use of these antennas for effective interstitial hyperthermia.