Early in the dark period of a 12-hr light, 12-hr dark synchronization regime cells of Chlamydomonas reinhardi are either in division or preparing for it, and are more sensitive to X and proton irradiation as expressed in terms of mortality and mechanisms of death exhibited by lethally injured cells. Irradiated and control cells were fixed for electron microscopic examination at intervals up to 5 days postexposure. X irradiation with 9000 R produced 69% mortality in 137c wild type cells and 71% mortality in the acetate-requiring strain ac-31. Ultrastructural responses observed in X-irradiated cells included nuclei with nonnucleolar condensations and swollen nuclear envelopes, nuclear envelope rupture in cells in an advanced state of disorganization, multinucleate cells and changes in chloroplasts and mitochondria. Irradiation of dividing 137c cells with 1.5-MeV protons possessing a dE/dx of $200\ {\rm MeV}\ {\rm cm}^{2}/{\rm g}$ produces 89% mortality following 9 krad and 98.3% following 18 krad. Proton-induced changes resembled those observed following X irradiation. Multiple nuclei per cell and variations in staining intensity of entire cells or parts of cells are documented. Two features absent in control cells were of special interest following both types of irradiation: (1) Cells were observed which remained joined in configurations closely resembling division profiles or division conformations. (2) Chromosomes and spindle fibers were present at a time not characteristic of the normal cell cycle.

Light-dark (L-D) synchronized Chlamydomonas reinhardi were X-irradiated (9108 R) early in the dark period as cells were preparing for or undergoing division. Division delay data were recorded both in terms of the entire population of cells and in terms of cells progressing to form either 4- or 8-celled clones. The lack of division delay exhibited by these cells is striking: The greatest delay among 4's was a 2.2-hr delay in their final division after irradiation at the 14th hour. The greatest delays in 8's were 2.3 and 2.4 hr in their final divisions after irradiation at the 13th and 14th hours. Such delays are small compared to delays induced in other cells by exposures leading to comparable survival levels (22% survival).

Light-dark (L-D) synchronized Chlamydomonas grow during a 12-hr light period and divide by a series of mitoses into 4 or 8 daughter cells during the early part of the following 12-hr dark period. Sensitivity to the lethal effects of 9108 R X-irradiation varies throughout the L-D cycle: Mortality rises from 20% at the 1st hour to 40% at the 9th hour, to 70% at the onset of the dark; it reaches a peak of about 85% at about the 14th hour, just before the first cytokinesis, and then returns to a level of about 45% when cell division has been completed (after data correction for multiplicity of targets per colony-forming unit). Most lethally affected cells complete at least one set of divisions (into 4 or 8 daughter cells) before they die; however, exposure shortly before the first nuclear division results in two sets of divisions before death, suggesting that these cells were committed in some way at the time of irradiation to divide again 24 hr later. Some single cells exposed prior to cytokinesis exhibit mixed-colony formation: About half of their progeny die and half survive, indicating that prior to cytokinesis there are perhaps two radiation-sensitive "targets" per cell.