The present work was designed to examine the effect of in vivo hyperthermia on the cell number and functions of polymorphonuclear leucocytes (PMN) and monocytes in human beings. Eight healthy volunteers were immersed into a waterbath (WI) (water temperature 39.5 degrees C) for 2 h, whereby their rectal temperature rose to 39.5 degrees C. On a later day they served as their own controls, being immersed into thermoneutral water (34.5 degrees C) for 2 h. Blood samples were collected before immersion, at body temperatures of 38, 39 and 39.5 degrees C as well as 2 h after water immersion. The neutrophil count was significantly increased at 39.5 degrees C, as well as 2 h after hot WI, compared with control. The monocyte count was significantly augmented at 38 and 39 degrees C and 2 h after hyperthermic load. The FMLP-induced chemiluminescence response, for a given number of PMN, was significantly reduced 2 h after hot WI. The total amount (per litre of blood) of superoxide production by PMN stimulated with opsonized zymosan (OZ) was significantly augmented at 39 and 39.5 degrees C and 2 h after WI. In vivo hyperthermia did not affect the function of monocytes, but when correlated to the changes in the concentrations of monocytes (response per litre blood) a significant increase in the phorbol myristate acetate (PMA)- and OZ-enhanced superoxide production occurred at 38 and 39 degrees C, as well as 2 h after termination of hot WI. Furthermore the OZ-enhanced monocyte chemiluminescence response per litre of blood was significantly enhanced 2 h after hot WI.(ABSTRACT TRUNCATED AT 250 WORDS)