Abstract. The main goal of this work is to derive useful diagnostic tools thathelp to stablish a link between star formation phenomena and AGN activity. Tothis purpose, we use the log[L(12µm)/L(60µm)] ratio ≡ L(12,60), the luminosityand equivalent width of the 7.7µm PAH line, the star formation rate (SFR) andthesize ofthe BRL. Ourresults show thatthese quantities, indeed, are intimatelyconnected, supporting the current view of a link between AGN and starburstactivity. 1. IntroductionThe mid-infrared spectrum (MIR) associated to an active galactic nucleus canbe explained in terms of the contribution of circumnuclear dust heated by thenuclear X-ray/UV/optical radiation, emission from the ionized gas and non-thermal emission from radio sources. In addition, the presence of nuclear andcircumnuclear starburst activity leaves important fingerprints in the spectrum inthe form of PAH emission features, nowdays considered as unambiguous proofs ofstar formation. Therefore, in the framework of the AGN-starburst connection, itis of outmost importance to quantify the star formation content of active galaxiesand cross-correlate it with robust indicators of the nuclear activity associated tothe central engine.2. Sample Data and AnalysisThe sample is composed by 14 IRLGs objects from Imanishi et al. (2000, 2006);41 Sy1 and 56 Sy2 galaxies from Imanishi (2003), Imanishi et al. (2004) andClavel J. et al. (2000); 24 ULIRGs Liner, 12 ULIRGs HII, 7 ULIRGs Sy1 and 7ULIRGs Sy2 from Imanishi et al. (2006, 2007) and Rigopoulou et al. (1999); 20Starburst galaxies from Brandl et al. (2006); 115 PG and 93 3CR objects fromSchweitzer et al. (2006) and Yong et al. (2007).The upper left panel in figure 1 shows the distribution of values of L(12,60)according to the type of object. The galaxies classified as Type 1 have a po-sitive, median value, in contrast with the rest of the sample, where the medianvalues indicates that the 60µm luminosity dominates (table 1). Adopting the72