For the first time, an antibody is used to detect the presence and to measure the concentration of ribulose-1,5-biphosphate carboxylase (RuBPCase) in several mutants of Euglena gracilis impaired in photosynthesis. Where present, the enzyme is correlated to its activity and to the chlorophyll content and photosynthetic CO2-fixation ability of the respective mutants. Dark-grown wild-type strains Z and bacillaris and mutants O1BS, O2BX, G1BU and P1BXL (which make normal sized chloroplasts with abnormal internal structure in the light) and Y1BXD (which does not develop the plastid beyond the proplastid stage in the light) each has about 0.1 × 10-11 μmol of active RuBPCase/cell. RuBPCase content increases in the light, but to a greater extent oin wild-type cells than in the above mutants, except possibly for mutant G1BU. The specific activity of the RuBPCase in vivo is estimated from the data to be about 1 nmol CO2-fixed min-1 (pmol RuBPCase)-1 in the light for both the wild-type and the above mutant strains and in the dark for the wild-type Z and some of the above mutant strains. RuBPCase was not detected in mutants Y3BUD, W34ZUD, SM-L1, W37HeHL and W3BUL, but was detected in mutant Y9ZNa1L which has no detectable protochlorophyll(ide) or chlorophyll(ide). In addition to recognizing both the large and small subunits of RuBPCase, the antibody also recognizes a polypeptide of > 60 000 daltons whose function is unknown.