Concentrations of the cosmogenic radioisotope 36Cl (t1/d= 3.0¿105 yr) in Antarctic ice and in rain and an upper limit to the seawater value have been determined using van de Graaff accelerator high energy mass spectrometry. Chlorine-36 concentrations in Antarctic ice lie in the range (2.5-8.7)¿106 atoms 36Cl/kg. Recent firn from Siple Station and ice samples collected from the Allan Hills ice field at locations where meteorites have been brought to the surface by glacial flow and ablation have 36Cl contents which vary by more than a factor of three. This variation can be attributed either to the effects of atmospheric mixing and scavenging or to radioactive decay in very old ice. The 36Cl concentration found in a sample of recent rainwater is much lower than has been reported in samples collected in the early 1960's, suggesting that there has been a large decrease in the concentration of atmospheric 36Cl derived from nuclear weapons tests over this time period. If measurement sensitivities can be increased somewhat, calculations indicate that the bomb spike of 36Cl may potentially be a useful oceanographic mixing tracer.