A group of glycolipid toxins, corynetoxin (CT), isolated from parasitized annual ryegrass, was shown to suppress the synthesis of both albumin and transferrin by cultured fetal rat hepatocytes. Based on [3H]leucine incorporation, inhibition of transferrin synthesis was greater than that of both albumin and total protein synthesis. As a result, the secretion of albumin and transferrin was decreased. The incorporation of [3H]N-AcGlc into cellular glycoproteins was only marginally affected by CT, although a dramatic reduction was observed with respect to the secreted proteins. Transferrin secreted into the culture medium was substantially non-glycosylated, judging by the absence of [3H]N-AcGlc. These studies suggested that the toxin preferentially affects the synthesis, and hence the secretion of glycoproteins, although it did not block the secretion of the proteins albumin and transferrin, as these did not accumulate intercellularly. Since transferrin labelled with [3H]leucine but not [3H]N-AcGlc is detected in the culture medium of hepatocytes exposed to CT, it was concluded that glycosylation of the protein is not required for secretion. This study shows that the effects of CT on protein synthesis and secretion in cultured hepatocytes are similar to those reported for tunicamycin (TM).