  We present the results of a multifrequency campaign from radio to hard X-ray
energies on the blazar 3C 279 during an optical high-state in January 2006. We
give the observational results (multifrequency light curves and spectra) and
compile an SED. This complements an SED from an optical low-state in June 2003.
Surprisingly the two SEDs differ only in their high-energy synchrotron emission
(near-IR - UV), while the low-energy inverse-Compton emission (X- to
Gamma-rays) remained unchanged. By interpreting with a steady-state leptonic
emission model, the variability among the SED can be reproduced by a change
solely of the low-energy cutoff of the relativistic electron distribution. In
an internal shock model for blazar emission, such a change could e.g. achieved
through a varying relative Lorentz factor of colliding shells producing
internal shocks in the jet.
