The effects of low-fluence exposures to (Pu, Be) neutrons (En = 4.2 MeV) have been studied in a sensitive human B-lymphoblastoid cell line, TK6. Mutations were scored for two genetic loci, hypoxanthine phosphoribosyltransferase (hgprt) and thymidine kinase (tk), as a function of dose and dose rate. For exposures limited to less than one cell cycle, the mutation frequency for the hgprt locus was 1.92 X 10(-7)/cGy. When exposures were protracted over multiple cell generations, mutation yields were increased to 6.07 X 10(-7)/cGy. Similar yields were obtained for the induction of tk-deficient mutants with a normal cell generation time (tk-ng) when exposures were carried out at very low dose rates over multiple cell generations. In the series of data presented here, the results obtained for short-duration neutron exposures are compared with data obtained for monoenergetic heavy charged particles of defined linear energy transfer (LET) produced at the BEVALAC accelerator at Lawrence Berkeley Laboratory. TK6 cells have been exposed to beams ranging in atomic number from 20Ne to 40Ar over an energy range from 330 to 670 MeV/amu. Mutation induction was evaluated for both loci for a subset of these beams. The results obtained with 20Ne ions of 425 MeV/amu (LET = 32 keV/microns) and 28Si ions of 670 MeV/amu (LET = 50 keV/microns) closely resemble the mutation yields obtained for brief exposures to (Pu, Be) neutrons. The nature of alterations in DNA structure induced within the tk locus of tk-ng mutants is reviewed for a series of neutron-induced mutants and a series of mutants induced by exposure to 40Ar ions (470 MeV/amu, LET = 95 keV/microns). The mutational spectra for these two types of mutants were similar and were dominated by allele loss mutations. Multilocus deletions inclusive of the c-erbA1 locus were common among tk-deficient mutants induced by these densely ionizing radiations. For the mutants induced by 40Ar ions, it is likely that the mutations were produced by the traversal of the chromosome by a single particle.