We study the law of a hypoelliptic Brownian motion on an infinitedimensional Heisenberg group based on an abstract Wiener space. We show that the endpoint distribution, which can be seen as a heat kernel measure, is absolutely continuous with respect to a certain product of Gaussian and Lebesgue measures, that the heat kernel is quasi-invariant under translation by the Cameron-Martin subgroup, and that the Radon-Nikodym derivative is Malliavin smooth.2010 Mathematics Subject Classification. Primary 58J35; Secondary 58J65, 60B15.