This study investigated the human gastrointestinal uptake (f(1)) and subsequent whole-body retention of orally administered inorganic radioactive cobalt. Of eight adult volunteers aged between 24 and 68 years, seven were given solutions of Co-57 (T-1/2 = 272 d) containing a stable cobalt carrier, and six were given carrier-free Co-58 (T-1/2 = 71 d). The administered activities ranged between 25 and 103 kBq. The observed mean A, based on 6 days accumulated urinary excretion sampling and whole-body counting, was 0.028 +/- 0.0048 for carrier-free Co-58, and 0.016 +/- 0.0021 for carrier-associated Co-57. These values were in reasonable agreement with values reported from previous studies involving a single intake of inorganic cobalt. The time pattern of the total retention (including residual cobalt in the GI tract) included a short-term component with a biological half-time of 0.71 +/- 0.03 d (average +/- 1 standard error of the mean for the two nuclides), an intermediate component with a mean half-time of 32 +/- 8.5 d, and a long-term component (observed in two volunteers) with half-times ranging from 80 to 720 d for the two isotopes. From the present data we conclude that for the short-lived Co-57 and Co-58, more than 95% of the internal absorbed dose was delivered within 7 days following oral intake; with a high individual variation influenced by the transit time of the unabsorbed cobalt through the gastro-intestinal tract. (C) 2015 Elsevier Ltd. All rights reserved.