1 The milk excretion of phenoxymethylpenicillin (PMP) was studied from both breasts in patients with mastitis (n = 12) and healthy volunteers (controls, n = 4) to investigate the hypothesis that milk transfer of PMP is higher in mastitic than in non-mastitic breasts. 2 Patients were included according to clinical symptoms of mastitis. Milk (and serum from controls) were sampled 0, 1, 2, 3, 4, 6 and 8 h after a single oral dose of 1320 mg PMP. Penicillin concentrations in milk and serum were measured by an agar diffusion technique. 3 Maximum milk concentrations (Cmax) of PMP in patients were higher (P less than 0.05) in mastitic than in non-mastitic breasts. The latter concentrations were higher (P less than 0.05) than those in breast milk from healthy controls. In milk from the mastitis patients (both breasts) the Cmax was reached after 2 h with a subsequent rapid decline in concentration. In milk from the healthy controls the PMP concentration reached a plateau after 4 h. The area under the milk concentration vs time curve (AUC0-8h) was not different for mastitic vs non-mastitic breast milk in patients nor for mastitic vs control breast milk. This can be explained by higher rates of appearance and disappearance of PMP in the breast milk of mastitis patients compared with healthy controls. In mastitic breast milk there was a moderate (P less than 0.01) increase in sodium and albumin compared with non-mastitic milk. However, milk potassium, glucose and lactose values were within normal limits.(ABSTRACT TRUNCATED AT 250 WORDS)