The kinetics of radiolabelled granulocytes in the reticuloendothelial system were studied in order to evaluate granulocyte margination in bone marrow. A total of 34 patients took part in a two-part study. In the first part, bone marrow uptake of indium-111-labelled granulocytes was retrospectively analysed in early (3-h) and late (24-h) images in 26 patients, 13 with bronchiectasis and 13 with enclosed abdominal abscesses. The ratios between early and late counts from the bone marrow, spleen, liver and inflammatory lesion were used to quantify granulocyte margination in bone marrow, postulating that if the lesion to bone marrow ratio at 24 h exceeds the value at 3 h, then the "excess" bone marrow counts on the early images would represent margination. In the second part, this suggestion was prospectively tested using Rutland-Patlak graphical and deconvolution analysis of dynamic data, acquired in 8 patients undergoing routine scanning with technetium-99m HMPAO-labelled granulocytes. In the first part of the study, it appeared that the bone marrow is a regional site of granulocyte margination, like the spleen, with at least one-half of the 3-h marrow signal arising from marginated granulocytes, compared with about two-thirds from the spleen. In the second part, it was found that the gradient of the Patlak plot, based on spleen and marrow, continuously decreased, consistent with bi-directional movement of cells between these organs and the blood. Granulocyte pooling in the marrow was confirmed with deconvolution analysis, which generated biphasic retention functions for marrow and spleen. These curves were also consistent with two-way granulocyte exchange, and gave mean cell transit times in both organs of about 12 min and probabilities of extraction on each pass of 5-10%. We conclude that granulocytes marginate in bone marrow to an extent similar to that in the spleen.