1997); (ii) whether the current star formation rate (SFR) of spiral galaxies belonging to a rich cluster is significantly lower than that of isolated galaxies of similar morphological type, as expected if either formation or evolutionary processes (e.g. ram-pressure) would contribute depleting the gaseous content of spiral galaxies in clusters, thus reducing the gaseous reservoir necessary to feed their star formation.

Kennicutt’s pioneering work in this field helped to establish that the massive ($\rm >10~M_{\sun}$), current ($\rm t<10^7~yrs$) star formation rate of disk galaxies is accurately traced by the integrated $\rm H\alpha$ + \[NII\] line intensity (Kennicutt 1983a) normalized to the underlying continuum intensity (Kennicutt 1989, 1990), i.e. by the line equivalent width (E.W.). More recently Kennicutt, Tamblyn & Congdon (1994, hereafter KTC94) determined that the ratio of the present to past rate increases from 0.01 to 1 along the Hubble sequence (from Sa to Irr), reflecting a change in the star formation properties of disks, and only secondarily a change in the bulge-to-disk ratio.

To investigate the hypothesis i), in Section 5.1 of this paper we study the dependence of the $\rm H\alpha$ E.W. on near-infrared H band luminosity. Our large (although not-complete) sample of galaxies spans a broad 