Cells and proteins of Chlamydomonas sp. (a common green alga in local reservoirs) were separated by ultrafiltration respectively into 3 fractions with variable molecular weights (MW: >100,10-3 and <3 kDa). After chlorination (20 degrees C, pH 7, Cl-2/DOC ratio of 20 mg Cl-2 mg(-1), 120 h), levels of disinfection by-products (DBPs) and mutagenicity (via Salmonella T100 mutation assay, -S9) were analyzed. The highest yields of chloroform (2571 mu mol C-1), DCAA (19,083 mu mol mol C-1) and TCAA (4939 mu mol mol C-1) were observed from the fraction of MW > 100 kDa, while the fraction of 3-10 kDa was potent DCAN precursor. In contrast, the chlorinated MW 3-10 kDa cell fraction showed high mutagenicity (maximum level of 93 rev mu L-1 at 2 min), while the MW > 100 kDa cell fraction showed low mutagenicity (maximum level of 16.6 rev mu L-1 at 7200 min) after chlorination. This indicated that unmeasured DBPs or possible interactions among the DBPs contributed to the mutagenicity. Comparing between the cell and protein fractions, the former was more potent in forming chloroform, DCAA, TCAA, DCAN and TCAN. This is the first study that fractionated algal cells and proteins were examined for DBP formation and mutagenicity. (C) 2012 Elsevier B.V. All rights reserved.