Caput femoris from 8 patients pre-operatively treated with clindamycin, and from 2 patients treated with lincomycin were investigated for antibacterial activity. Using the agar diffusion method, the concentrations of clindániycin ranged from 0.4 to 4.9 μg/g, and the ratio bone/serum concentration was 0.1 to 0.5. The concentration of lincomycin was 1.7 and 2.9 μg/g. Using the electrophoretic method higher levels of clindamycin were measured, possibly as a consequence of a diminu tion in the binding capacity of the bone. This phenomenon was more marked with lincomycin.