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Welding fumes generated by manual metal arc welding (MMAW) with an automatic welding machine were extracted with water and both the soluble and insoluble fractions were investigated for mitotic delay and for sister chromatid exchange (SCE) induction in cultured Chinese hamster lung (Don) cells. Five flux-coated electrodes of varying composition were chosen as representative of those available. In general, water-soluble and water-insoluble fractions induced SCE in proportion to their chromium(VI) content, and any contribution from chromium(III), fluorides, nickel, manganese or other fume constituents was minor. However, mitotic delay could not be explained in terms of the chromium(VI) concentration alone. This indicated that other components of both the soluble and insoluble fractions of the fumes were capable of inducing mitotic delay. The activity of insoluble fume fractions, either in mitotic delay- or in SCE-induction, was markedly less than that of soluble fractions. This was presumably because of the lower, though still significant, bioavailability of 'insoluble' components of the fume particles.