Dimethyl disulphide, a breakdown product of the amino acid S-methylcysteine sulphoxide present in high concentrations in the Brassica family, was fed to a group of adult fowls and the effects were studied haematologically and ultrastructurally. By the seventh day of the experiment Heinz bodies had appeared in most of the red blood cells and between the eighth and 12th days the birds showed leg weakness, feather ruffling and lethargy. The Heinz bodies were divided into two different morphological forms and many vacuoles were also present in these cells. The haemoglobin levels and packed cell volumes were significantly lower that those of the controls and in turn the mean cell haemoglobin and especially the mean cell haemoglobin concentration values were reduced.