Caenorhabditis elegans is being used as a model system for the evaluation of ionizing and nonionizing radiation effects on cell reproduction, differentiation and mutation in vivo. Fluence/dose vs response and quality factor (RBE) vs LET (linear energy transfer) relationships have been constructed for the following biological effects using gamma rays, accelerated particles and ultraviolet light.Cell inactivation. Radiation damage to a four-cell gonad precursor interferes with the normal developmental program that constructs an adult gonad whose normal function is to produce 280 offspring by self fertilization. This provides a measure of altered gene expression and cell inactivation.Mutation. Recessive lethal mutations in a set of over 300 essential genes are being isolated using a balancer chromosome technique and the spectrum of mutant structures is being evaluated as a function of radiation type.Chromosome Rearrangement. The production of duplications of the right arm of the X chromosome has been measured. The kinetics indicate that the production rate is the result of competing processes of sperm inactivation and chromosome damage. The formation of polycentric chromosomes in intestinal nuclei has also been evaluated.Damage Repair. Radiation and mutagen hypersensitive mutant strains have been tested. The patterns of sensitivity with respect to radiation type and biological effect indicate 1) that a complex repair pathway is present in the nematode and 2) that such mutants can be successfully used to “tune” the sensitivity of the system to specific mutagens/radiation types.