In vitro toxicity studies were initiated in order to determine if chlorination affects vascular endothelial cells. Twelfth to twentieth passage porcine aortic vascular endothelial cells (PAE) were grown to confluency and replated in the presence of complete media (Eagle's minimum essential media supplemented with 20% fetal bovine serum) which had been preincubated for 30 minutes with 15.0 mg/liter chlorine. During a 72-hour exposure period, control PAE cells grew to confluency, an increase of approximately 9 fold in the number of cells/plate. Those cells exposed to media preincubated with 15.0 mg/liter chlorine derived from sodium hypochlorite increased only 6 fold. There was no sign of cell killing, but an apparent inhibition of cell division. No effect was seen when either the amino acid or vitamin component of the complete media was reacted at the 15.0 mg dose level. However, when the serum component was preincubated with 15.0 mg chlorine/liter as sodium hypochlorite, an inhibition in growth rate similar to the complete media occurred.