Results of the 14C measurements in atmospheric CO2 in the first half of 1986 are presented. CO2 samples were systematically collected in Krakow in two-week cycles and, after conversion to benzene, measured in a liquid scintillation spectrometer. 14C activity and 13C/12C ratio are reported as δ14C and δ13CPDB, respectively. For about three weeks after April 26, 1986 (the Chernobyl accident) an increase of ∼9% above the normal level for Krakow was observed. A rough estimate of the 14C release to the lower atmosphere during the accident gave a value 900 Ci, which is ∼1.8 × 10−5 of the total activity released to the atmosphere.