A barley mutant RPr84/90 has been isolated by selecting for plants which grow poorly in natural air, but normally in air enriched to 0.8% CO2. After 5 minutes of photosynthesis in air containing(14)CO2 this mutant incorporated 26% of the(14)C carbon into phosphoglycollate, a compound not normally labelled in wild type (cv. Maris Mink) leaves.The activity of phosphoglycollate phosphatase (EC 3.1.1.18) was 1.2 nkat mg(-1) protein at 30°C in RPr 84/90 compared to 19.2 nkat mg(-1) protein in the wild-type leaves. Phosphoglycollate phosphatase activity was not detected after protein separation by electrophoresis of leaf extracts from the mutant on polyacrylamide gels; on linear 5% acrylamide gels three bands with enzyme activity were separated from extracts of wild type plants. Gradient gel electrophoresis followed by activity staining showed two bands in Maris Mink tracks of MW 86,000 and 96,000, but no bands in 84/90. This is the first report of isozymes of phosphoglycollate phosphatase in barley which were absent in the mutant extracts. Our results confirm an earlier report of isozymes of this phosphatase in Phaseolus vulgaris [18].The photosynthetic rate of RPr 84/90 in 1% O2, 350 μl CO2 l(-1) was 9-12 mg CO2 dm(-2) h(-1) at 20°C, whereas the wild-type rate was 27-29 mg CO2 dm(-2) h(-1) at 20°C. In 21% O2, 350 μl CO2 l(-1) the rate was 2-3 mg CO2 dm(-2) h(-1) in the mutant and 20 mg CO2 dm(-2) h(-1) in the wild type.Genetic analysis has shown that the mutation segregates as a single recessive nuclear gene.