The pattern of changes in neutrophil myeloperoxidase (MPO) before, during and after bacteraemia was studied in 34 patients recovering from autologous bone marrow transplantation for relapsed Hodgkin's disease and non Hodgkin's lymphomas. Thirteen patients received haemopoietic growth factors (7 received M-CSF, 3 received G-CSF and 3 GM-CSF). The mean peroxidase index (MPXI) produced as part of a routine FBC performed by a flow cytochemistry blood autoanalyser (Technicon H*1) was used as a parameter to assess the MPO and subsequently the azurophil degranulation. The manufacturer's normal values for MPXI range from -10 to +10. Median MPXI on the day of documented bacteraemia was just below normal in the control and M-CSF groups (-10.8 and -8.9 respectively), but it was much below normal in the G-CSF (-16.5, P < 0.05) and even lower in the GM-CSF group (-39.6, P < 0.02); this correlated well with the decreased bacteraemia incidence in the last two groups. Although contact of neutrophils with bacterial chemoattractants resulted in primary degranulation in all groups, the pattern of changes in MPO content was different, suggesting that neutrophils primed in vivo with various haemopoietins respond to the challenge of microbial agents via different pathways.