The techniques for alpha-particle autoradiography based on the plastic nuclear track detector CR-39, previously reported, have been developed considerably. The techniques are applied to alpha-autoradiography of human lung tissue in particular but are applicable to any biological tissue. The most important developments are: (i) Improvements in the manufacture and pre-etching of the plastic. These allow activities as low as approximately 10(-15) Ci g-1 to be determined. (ii) High resolution alpha-particle spectroscopy in CR-39 plastic based on the analysis of the structure of the etched track. This enables the energy of individual alpha-particles to be determined to approximately 35 keV. (iii) Calculation of the effective thickness of tissue sampled by the plastic. This relates the tissue activity to the track density on the plastic. (iv) A deconvolution analysis which takes the distributions of track length and dip angle in the plastic and determines the alpha-particle range spectrum and distribution of tissue activity with height above the plastic surface. This enables both the absolute abundance and the microdistribution of alpha-active nuclides present to be determined. (v) The analysis of radon diffusion in tissue to determine the mean radon diffusion distance in tissue and plastic.