Perhaps also of interest is the `drc' package on CRAN. There's an article in JSS describing it (probably the same as the package vignette--- haven't checked).
From: Prof Brian Ripley
> On Thu, 2 Feb 2006 Arne.Muller@sanofi-aventis.com wrote:
> > I was wondering if there is an R-package to automatically
> calculate the
> > IC50 value (concentration of a substrance that inhibits
> cell growth to
> > 50%) for some measurements.
> Function dose.p in recommended package MASS.
> Brian D. Ripley, firstname.lastname@example.org
> Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
> University of Oxford, Tel: +44 1865 272861 (self)
> 1 South Parks Road, +44 1865 272866 (PA)
> Oxford OX1 3TG, UK Fax: +44 1865 272595
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