An attempt is made to analyse the structure ofS-matrix elements for multiple production by phenomenological considerations based upon the two center model. We start from two assumptions: 1) the momentum distributions of pions emitted in the for- and backward cones are isotropic in each center-of-mass system (the fire ball rest system) and the average values ɛ0 of the pion energies in this system do not depend on the details of collisions; 2) the mean value of the momentum transferΔ is always of the order of the nucleon massM. From these assumptions the following results are derived in Sect.2: 1) the multiplicity is given byn⋍ξ√2E0|Δ|/ɛ0, where ξ is the ratio ℳ/ℳmax of the fire ball mass ℳ and its maximum value ℳmax. Experimentally, ξ⋍0.5 at 1013 eV; 2) the lower limit of the inelasticityK is given byK⩾ξ2. At 1013 eV we haveK⩾ξ2⋍0.2 which is consistent with experiments. The relation between multiplicity and inelasticity is discussed; 3) in the center-of-mass system of the fire ball and the recoil nucleon (« isobar system » IBS), the ratio |M|/ℳ of the momentum of the nucleonM and the fire ball mass ℳ is also expressed by the parameter ξ as |N|/ℳ=(1−ξ2)/2ξ. At 1013 eV we have |M|/ℳ⋍1. This means that an equipartition of energy is approximately valid for the three possible degrees of freedom: the motion of the recoil nucleon, the translational and internal motion of the fire ball. |M|≪ℳ is excluded, unlessK is unity, contrary to experiments; 4) when the equipartition holds, the mean value of the transverse momentum is constant and its value is of the order of ɛ0. In Sect.3, the ℳ andn-dependence of theS-matrix is discussed. And by using the corresponding results we estimate the asymmetry in numbers of particlesn1 andn2 emitted in the for- and backward cones. The parameter [〈(n1−n2)2〉/〈(n1+n2)2〉]1/2, a measure of the asymmetry, becomes 1/3 due to statistical fluctuation alone. It becomes larger if the energy transferΔ0 is not zero. Discussion is also made of the condition by which the average value ɛ0 of the pion energy in the fire ball is constant. The properties of the fire ball seem to be independent from the mechanism of its production. Extending this point of view, the similarity between the multiple production inN-N collisions andN-N annihilation is discussed in Sect.4. Conclusions are given in Sect.5.