Improvements in the argon laser microbeam have made it possible to cause damage to chromosomes of tissue culture cells without prior treatment of the cells with a photosensitizing agent. These results have been confirmed independently in two laboratories.

An argon laser microbeam was used to irradiate regions outside and inside the secondary constrictions of nucleolar organizing chromosomes. Irradiation immediately adjacent to the constriction consistently resulted in the loss of nucleolar organizing capacity. Xrradiation 2 pm down the chromosome from the secondary constriction did not affect the capacity to organize a nucleolus. Irradiation directly inside the secondary constriction did not affect the ability to organize a nucleolus in 50 % of the cases. These data are discussed in relation to current ideas that secondary constrictions are nucleolus organizers. Alternative models are presented.

Cells of the established cell lines of the rat kangaroo (Puotorous tridactylis), were laser microirradiated in order to study their nucleolar organizers. Microbeam irradiation of prophase and metaphase nucleolar organizer sites in both male (PTK 1) and female (PTK2) cells resulted in a reduction in the number of normal nucleoli formed following mitosis. Irradiation of the one nucleolar organizer in the male cells, often resulted in the formation of numerous micronucleoli. Micronucleoli were also formed in small, extra nuclei, that were probably formed as a result of microirradiation of the centromere region of autosomes. Preliminary cytochemical analysis of the micronucleoli is presented and the possible mechanism of the formation is discussed.

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