The results of a comparative study of heavy particles of interest in radiotherapy are reported in four parts. In this Part IV, early skin reactions and late reactions (foot deformity) in mice for various heavy particles are reported. For heavy charged particles, the exposures were made at the entrance region (plateau) and centre of the peak (10 cm wide peaks). For 60Co gamma rays and fast neutrons (50 MeV D leads to Be), the exposures were made at the peak of the depth-dose curve. The time-course of development of skin reaction and subsequent healing after exposure to heavy ions or 60Co gamma rays were remarkably similar, suggesting that skin damage and subsequent epithelial repopulation after exposure to heavy ions are not different from 60Co gamma rays. When the Bragg peaks were broadened to 10 cm, the RBE at the peak, compared with the entrance region, was significantly higher for carbon ions but nearly the same or even lower for neon and argon ions because of saturation effects at high LET. The RBE for fast neutrons was comparable to that at the peak for carbon ions. The correlation between early skin reaction and foot deformity remained the same for all particles.