We analyse an age-structured model for haematopoiesis, describing the devel
opment of specialized cells in the blood from undifferentiated stem cells a
nd including the controlling effects of hormones. Variation in the length o
f time for maturing of precursor cells in this model has a stabilizing infl
uence. When the maturing process does not vary, then the age-structured mod
el reduces to a delay differential equation. Depending on the death process
considered, either a differential equation with two time delays or a diffe
rential equation with a state-dependent delay is obtained. Each of these is
analysed in turn, for its linear stability. A sensitivity analysis of the
parameters in this model shows which biochemical processes in the negative
feedback most strongly affect the solutions.