A man aged 58 was admitted to our hospital with periodic diarrhea for the past 6 years. On admission, he was well-developed and well-nourished, but looked somewhat ill. Isospora belli and Giardia lamblia were found in his warm liquid stool. He was treated with Atebrin ® (quinaerine hydrochloride) following the diagnosis of protozoa infection. Hematologic examination revealed the following values: hemoglobin 14.0 g/100 ml, hematocrit 43.5%, RBC 4710000/mm 3, platelets 235000, white blood cells 14800 (lymphocytes 56.80/0 with 1.2°/0 of atypical lymphocytes and 10.8~o of binuclear lymphoeytes). One or two small lymphnodes were palpable in the right supraclavicular region, and a biopsy specimen showed chronic lymphadenitis. On physical and other laboratory examinations there were no findings suggestive of malignancy. Chromosome study was undertaken three times before and after therapy. The peripheral leukocytes were cultured by the short-term culture method after Hungerfold (1965). The data are shown in Table 1. Before treatment there were no chromosomal abnormalities. After the administration of Atebrin ® (0.3 g for 14 days) the chromosome number ranged from 43 to 49 with a modal number of 48. In the cells with 48 chromosomes (Fig. 1) an extra chromosome No. 3, a missing chromosome No. 2 and two morphologically well-defined abnormal chromosomes were recognized. In the second sampling, an abnormal karyotype with one or two marker chromosomes was observed in high frequency (79.5%). It was also found in the third sampling, which was carried out 2 months after withdrawal of Atebrin ®. However, the frequency of the abnormal karyotype was low (34.5~o), and normal diploid cells were increased in number.