As a consequence of 241Am contamination in a German industrial institute extensive in-vivo measurements have been performed in the lung counter of the Karlsruhe Nuclear Research Center (KfK) starting in 1986. Because of poor energy resolution of phoswich detectors, measurements have been severely disturbed by Cesium body burdens due to the Chernobyl reactor accident. After development of a special background prediction method measurements showed the 241Am activity to be distributed over lungs, liver and skeleton. For assessment of respective 241Am organ burdens extensive calibration measurements have been performed using the Lawrence Livermore National Laboratory chest phantom and some bone phantoms from the New York University Medical Center. From April to June, 1987 about 130 involved persons were screened in the KfK lung counter with the detectors located over the lungs, this being about 150 Bq. In October, 1987 some of these persons have been counted in four different measuring geometries with the detectors over head, lungs, liver and knees, respectively. The ratio of estimated liver and skeleton burdens was found to be unity within limits of error. Using this result as boundary condition for evaluation of other measurements, the biological half-life of the pulmonary compartment was found to be about 200 d. For further evaluations the ICRP lung model parameters were interpolated for biological half-life of 200 d. Interpolated parameters were used to calculate the time dependence of the 241Am organ burdens after a single intake by inhalation of aerosols with different particle size distributions. The results of these calculations were found to be in good agreement with measurements when using a mean particle size of 1 µm AMAD. From March to July, 1987 some persons have been treated with DTPA. The measurements of these persons clearly indicate that 241Am activity is removed primarily from the liver rather than from the skeleton.