  In order to explain the spectroscopic observations of most of the galaxies in
intermediate redshift clusters, bursts of star formation superimposed to the
traditional scenario of galactic evolution are needed. The analysis of spectral
lines and colours by means of an evolutionary synthesis model, including both
the stellar contribution and the emission of the ionized gas, allows in most of
the cases the determination of the time elapsed since the end of the burst and
the fraction of galactic mass involved in it. In the four clusters considered
(AC103, AC114, AC118 at $\rm z=0.31$ and Cl1358+6245 at $\rm z=0.33$), the
theoretical analysis demonstrates that the bursts affect substantial galactic
mass fractions, typically 30 \% or more. The observations can be equally well
reproduced by either elliptical+burst models or by spiral+burst models in which
the star formation is truncated at the end of the burst. A way to determine the
galactic original type is suggested.
