Motivated by the difference between SLD's recent measurement of ALR and the
corresponding LEP results, we explore which kinds of new particles can (1)
contribute dominantly to new physics through oblique corrections, (2) produce
negative values for S and T, and (3) not be in conflict with any other
experiments, on or off the Z resonance. We are typically led to models which
involve new particles which are not much heavier than MZ/2, and so which may
also have implications for other experiments in the near future. For such light
particles, we show how the oblique-parameter analysis of purely Z-pole data
requires the interpretation of the data in terms of modified parameters, S' and
T', whose difference from S and T improves the available parameter space of the
models.