Little is known of the biological effects and human occupational exposure of the lanthanides. A case of a photoengraver professionally exposed to cored arc light carbon fumes droped with Ce was studied to establish whether the observed pulmonary alterations were related to the exposure to rare earths present in cored arc light carbon. Neutron activation analysis (NAA) has been used for the determination of rare earths in lung and lymph node biopsies of the worker as well as a control group. The results show a clearly abnormal presence of rare earths in the biopsies of the worker. The pulmonary concentrations of the worker for La, Ce, Nd and Sm were 45.6, 166.5, 57.5 and 4.55 ppm respectively whereas the corresponding concentrations of the controls were 0.017, 0.071, 0.046 and 0.0025. The differences in Eu, Tb, Yb and Lu concentrations were lower than these but were consistently higher than the values in the lungs of the control group. The amounts of La, Ce, Nd and Sm in lymph nodes of the worker were also significantly higher than in the controls. The differences, however, are lower when compared with the lung. Results on Eu, Tb, Yb and Lu in lymph nodes are judged as not being sufficiently definite to establish differences between the worker and the control group. These findings appear particularly important if correlated to the severe pneumoconiosis in the worker as substantiated by clinical and chest X-ray analysis. Determination of 20 elements, besides the rare earths, in the lung and lymph nodes of the subject examined shows no significative differences as compared to the control group. The estimated radiological dose due to the natural thorium, which is generally present as impurity of rare-earth compounds, and the radioactive, naturally-occurring 138La, 144+150Nd and 147Sm tends to exclude the effect of ionizing radiation in the pathogenesis of lung fibrosis. The concentrations of RE in lungs of heavy smokers were also compared with a group of non-smokers. The high accumulation of rare earths in lungs and lymph nodes and the lung fibrosis of the worker examined indicates a possible long-term risk of rare-earth pneumoconiosis in occupational workers. The relationship between the obvious accumulation of rare earths in human lungs under chronic exposure and the interstitial pneumoconiosis calls attention to proposals for MPC (maximum permissible concentration) limits of occupational exposure to rare earths in air in order to protect workers exposed to respirable rare-earth dusts.