Dwarf colony mutants of staphylococci were isolated from per operative samplings obtained from three patients with a bacterial contamination of their osteosynthesis material. The dwarf mutants grown in trypticase soja agar (TSA) after 72 h at 37 degrees C (two cases) and in nutrient broth (one case). They displayed poor growth (very small, transparent colonies on TSA), reduced biochemical activity, deficient pigment formation and intermediary resistance to aminoglucoside antibiotics (kanamycin, streptomycin, gentamicin). Biochemically, two types of metabolic defects have been demonstrated among these dwarf strains. Two of them are vitamin K2-less (strains 30 and 73) and are supplemented by menadion natrium bisulfit at concentrations ranging from 0.2 to 10 mug per ml. In these cases the dwarf colony mutants coexisted with normal colony of S. epidermidis (case 30) and S. aureus (case 73) belonging to serotypes 52260 and I respectively. The third strain (case 163) supplemented by horse blood is hemin-requiring. This strain could be characterized in media supplemented with blood, because it was isolated only as dwarf colony mutant from the patient; it belongs to serotype 52260 (S. epidermidis). This hemin-less dwarf mutant is the single responsible of the pathological manifestations of the patient 163. In the cases of vitamin K12-less mutants (strains 30 and 73) the coexistence of dwarf mutants and normal colonies of the same serotype may make possible the elucidation of relapse in infectious complications of the patients with osteosynthesis.