Many of the current passive microwave rain-rate retrieval methods rely on databases composed of simulated brightness temperatures. Radiative transfer models generate these databases using parameterized raindrop size distributions, such as the Marshall-Palmer distribution, to describe precipitating hydrometeors. During a series of field experiments conducted in support of the Tropical Rainfall Measurement Mission (TRMM) ground validation program, airborne sensors measured drop size distributions at a variety of tropical locations. In this work, we compare measured drop size distributions with various well-known parameterizations. Best approximations to the actual drop size distributions in each vertical layer are used as input to a radiative transfer model. Calculated microwave brightness temperatures are compared to those obtained by assuming standard drop size distribution parameterizations in an attempt to quantify the uncertainties in brightness temperature databases resulting from use of common parameterization methods.