We present a laboratory version of a photoacoustic mammoscope, based on a parallel plate geometry. The instrument is built around a flat high-density ultrasound detector matrix. The light source is a Q-switched Nd:YAG laser with a pulse duration of 5 ns. To test the instrument, a novel photoacoustic phantom is developed using poly(vinyl alcohol) gel, prepared by a simple procedure that imparts optical scattering suggestive of breast tissue to it without the requirement for extraneous scattering particles. Tumor simulating poly(vinyl alcohol) gel spheres appropriately dyed at the time of preparation are characterized for optical absorption coefficients. These are then embedded in the phantom to serve as tumors with absorption contrasts ranging from 2 to 7, with respect to the background. Photoacoustic studies in transmission mode are performed, by acquiring the laser-induced ultrasound signals from regions of interest in the phantom. Image reconstruction is based on a delay-and-sum beamforming algorithm. The results of these studies provide an insight into the capabilities of the prototype. Various recommendations that will guide the evolving of our laboratory prototype into a clinical version are also discussed. © 2004 Society of Photo-Optical Instrumentation Engineers.