Total- and free carnitine content of washed pooled red blood cells collected from five children prior to and on the last day of combined pivampicillin and equal molar carnitine treatment were measured. On the last day of treatment (day 7) the level of total carnitine decreased from 47.5 +/- 3.39 to 37.5 +/- 2.48 nmol/ml, mean +/- SEM (p less than 0.05) with a concomitant decrease of free carnitine (from 19.2 +/- 0.97 to 15.5 +/- 0.99 nmol/ml, p less than 0.05) as compared with the pretreatment control day (day 0). The calculated amount of acid soluble carnitine esters also fell (from 28.2 +/- 3.38 to 21.9 +/- 1.78 nmol/ml). The same effects were found when the carnitine levels were referred to haemoglobin or water content of samples. These results demonstrate that in pivampicillin treatment the carnitine pool of erythrocytes also alters. In agreement with previous findings the data presented here suggest, that the administered carnitine was not sufficient to meet the enhanced needs of the organism caused by the pivalate load and that the organism utilized some of its stores for pivaloylcarnitine production. The decreased carnitine ester level of erythrocytes suggest, that the red blood cells do not participate in significant extent in pivaloylcarnitine transport or production.