Ciprofloxacin serum levels were measured after administration of the drug to 36 patients with septicemia (at least one positive blood culture) who were able to take oral medication. Patients were randomly allocated to receive ciprofloxacin 500 mg p.o. (n = 21) or 200 mg i.v. over 30 min (n = 15). A first dose was administered 18-30 h after the last positive blood culture (day 1), and a second dose four days later (day 5) in some patients. In addition to ciprofloxacin, standard antibiotics were administered. Organisms isolated were Escherichia coli (15), other gram-negative bacteria (6), Streptococcus pneumoniae (7), Staphylococcus aureus (2), and other gram-positive bacteria (6). None of the patients vomited. Ciprofloxacin serum concentrations 1 h after oral administration were in the range 0.09-2.32 mg/l, and 2 h after administration in the range 0.5-7.27 mg/l. The average terminal half-life was 8.6 h. In individual patients serum concentrations and area-under-the-curve values were compared. Poor correlation was found between values measured on day 1 and day 5 after oral administration, whereas the correlation was excellent after i.v. administration. Serum levels 2 h after oral administration were 30-900 times the MICs for the gram-negative organisms, but were in the range of the MICs for the gram-positive organisms in some cases. In conclusion, ciprofloxacin serum levels are difficult to predict in septicemia patients after oral administration, but probably suffice to treat infections caused by gram-negative organisms.