Low-energy gamma-emitting isotopes encapsulated for permanent implant are routinely applied in brachytherapy, most notably for prostate cancer. Before clinical use of a new source design, a full dosimetric analysis and standardized calibration are essential. Results of experimental measurement and analysis are reported here for the I-Plant (Implant Sciences Corporation) 125I source, model 3500. Dose measurements were made using standard methods employing thermoluminscent dosimeters in a water equivalent plastic phantom. Precision machined bores in the phantom located dosimeters and source(s) in a reproducible fixed geometry providing for transverse-axis and angular dose profiles over a range of distances from 0.17 to 10 cm. The data were analyzed in terms of parameters recommended by AAPM TG-43. The dose-rate constant, delta = 1.01 cGy/h U (+/-6%) (1 U = 1 cGy cm2 h(-1)), was evaluated with reference to a TG-51 calibrated 60Co standard, accounting for dosimeter response differences between 60Co and 125I photons. The radial dose function, g(r), the anisotropy function, F(r, theta), the anisotropy factor, phi(an)(r), and the point-source approximation anisotropy constant, phi(an), were derived from one- and two-dimensional dose distribution data measured in the phantom, accounting for finite dosimeter volume and with attention to inter-chip effects. The results confirm prior dosimetric characterization of the model 3500, and indicate that the new source is comparable to the MED3631-A/M and 6702 source designs and may substitute for model 6711 in permanent implants for the treatment of prostate cancer.