  Context: Very high magnetic fields at the surface of neutron stars or in the
accretion disk of black holes inhibit the production of jets. Aims: We quantify
here the magnetic field strength for jet formation. Methods: By using the
Alfven Radius, R_A, we study what we call {\it the basic condition}, R_A/R_*=1
or R_A/R_{LSO}=1 (LSO, last stable orbit), in its dependency on the magnetic
field strength and the mass accretion rate, and we analyse these results in 3-D
and 2-D plots in the case of neutron star and black hole accretor systems,
respectively. For this purpose, we did a systematic search of all available
observational data for magnetic field strength and the mass accretion rate.
Results: The association of a classical X-ray pulsar (i.e. B ~10^{12} G) with
jets is excluded even if accreting at the Eddington critical rate. Z-sources
may develop jets for B \simlt 10^{8.2} G, whereas Atoll-sources are potential
sources of jets if B \simlt 10^{7.7} G. It is not ruled out that a millisecond
X-ray pulsar could develop jets, at least for those sources where B \simlt
10^{7.5} G. In this case the millisecond X-ray pulsar could switch to a
microquasar phase during its maximum accretion rate. For stellar-mass black
hole X-ray binaries, the condition is that B \simlt 1.35 x 10^8 G and B \simlt
5 x 10^8 G at the last stable orbit for a Schwarzschild and a Kerr black hole,
respectively. For active galactic nuclei (AGNs), it reaches B \simlt 10^{5.9} G
for each kind of black hole. These theoretical results are in complete
agreement with available observational data.
