For cytogenetic analysis of chromosome abnormalities, we have developed an instrument which can produce semi-automatically chromosome slides for microscopic observation. This instrument has automatic slide supplier and slide transfer system. Chromosome slides were obtained by the steam dry method; slides were sequentially exposed to different atmosphere controlled strictly in respect of temperature and humidity. Slides with well-spread chromosomes were obtained constantly in quality and rapidly in speed of 5 slides per minute. Precise controls of room temperature and humidity were attained by the use of 26 thermomodules. Accuracy of the controls was excellently high, showing a fluctuation of +/- 0.5 degrees C in temperature and +/- 3% in humidity for any setting in the use range. This instrument was proved to be very useful for rapid obtaining of a large number of chromosome slides with highly qualified metaphase spreads. This instrument will be utilized to assist the automation analysis system of chromosome abnormalities. Mechanism of chromosome spreading, problems in actual use, and future improvements were discussed.