Biosynthesis of ferritin subunits by cell sets isolated from normal human peripheral blood, spleens of Hodgkin's disease patients, and tumor cell lines were investigated. Normal mature hematopoietic cells made a ferritin with more H (21K) than L (19K) subunits. The reverse was found for a promyelocytic tumor cell line and tumor cell lines derived from other tissues. Two dimensional electrophoresis indicated H has a lower pI than L. Therefore relative proportions of the two subunits contribute to the electrophoretically distinct forms of the isoferritins. In response to increasing concentrations of iron in vitro, a selected monocyte population synthesized more H than L; L biosynthesis however increased more than H. Some possible regulatory implications of these observations are discussed.