Abstract: Chloroplasts with different chlorophyll a/b ratios were isolated from 7 to 8 days old wheat seedlings and the activities of reduction of 2,6-dichlorophenol-indophenol (DPIP) by these chloroplasts as function of the chlorophyll a/b ratios were studied under mono-chromatic light (650 m μ, 670 mμ, 680 mμ, 707 mμ). It was found that the DPIP reducing activities by these chloroplasts varied with their chlorophyll a/b ratios, and these variations are affected by the wavelengths of the illuminating light. Under 650 mμ, at the a/b ratios of 2.2 to 2.82, the activities of DPIP reduction in- creased with the a/b ratios, but decreased when the a/b ratios were higher than 2.82. Under 670 mμ, the DPIP reducing activities also varied with the a/b ratios of isolated chloroplasts. However, the variation was more gradual and steadier. Under 680 mμ, the DPIP reducing activities increased with the a/b ratios over 3.0, but decreased rather suddenly at a/b ratio of 3.30. Essentially the same relation held for 707 mμ, but the Hill reaction activities ceased to decline farther when a/b ratio rose to 3.40 at 707 mμ. When the results were analyzed in terms of the “relative activities” of the chloroplasts of the above mentioned wave lengths, it was found that the values of the "relative activity" (a/a+b) declined steadily from 1.47–1.29 at an a/b ratio range of 2.05–3.40, while the values of the "relative activity" (b/a+b)increased steadily from 3.10--4.40 at the same range of a/b ratios. But it is to be noted that, under 650 mμ, the (a/a+b) was 1.36 at the a/b ratio range of 2.63–2.82. Interesting enough, the DPIP reducing activities were the highest of all with these a/b ratios. Activities of DPIP reduction by isolated chloroplasts kept at 0 ℃, 20 ℃, 30 ℃, and 45 ℃ diminished with time when illuminated at all wavelengths. However, for those kept at 45 ℃ their activities were lost after 20 minutes, except those illuminated with the wave length of 680 mμ which still maintained 30 % of the initial activity. When kept at the above mentioned temperatures, the chlorophyll a/b ratios of all batches of chloroplasts declined steadily with time. The above results are interpreted as being indicative of the possibility that the pigment systems for the partial reaction (Hill reaction) of the over-all photosynthesis process consisted mainly of Chlb650 and Chla670. And the correlation of temperature and abolition of activity reduction of DPIP of chloroplasts varied under monochromatic light of different wave lengths.