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The two-pion production in pp-collisions has been investigated at CELSIUS inexclusive measurements from threshold up to $T_p$ = 1.36 GeV. Total anddifferential cross sections have been obtained for the channels $pn\pi^+\pi^0$,$pp\pi^+\pi^-$, $pp\pi^0\pi^0$ and also $nn\pi^+\pi^+$. For intermediateincident energies $T_p >$ 1 GeV, i.e. in the region which is beyond the Roperexcitation but at the onset of $\Delta\Delta$ excitation, the total$pp\pi^0\pi^0$ cross section falls behind theoretical predictions by as much asan order of magnitude near 1.2 GeV, whereas the $nn\pi^+\pi^+$ cross section isa factor of five larger than predicted. An isospin decompostion of the totalcross sections exhibits a s-channel-like energy dependence in the region of theRoper excitation as well as a significant contribution of an isospin 3/2resonance other than the $\Delta(1232)$. As possible candidates the$\Delta(1600)$ and the $\Delta(1700)$ are discussed.

Two-Pion Production in Proton-Proton Collisions -- Experimental Total Cross Sections and their Isospin Decomposition