The present study was undertaken to evaluate the relative bioavailability of doxycycline hyclate and doxycycline-L-methylenelysine, a new synthetic derivative of doxycycline. Single doses, equivalent to 100mg of doxycycline hyclate, were administered orally to 10 healthy male volunteers. Serum concentrations were determined periodically over 12 hrs. by microbiological assay. Peak serum concentrations were reached within 4 hrs. Doxycycline hyclate $(1.38mcg.ml^{-1})$ gave 1.66 tines higher concentration than doxycycline-L-methylenelysine $(0.38mcg.;ml^{-1})$. And the areas under the serum concentrationtime curves 0 to 12 hrs were $8.38mcg.hr.ml^{-1}$ (doxycycline hyclate) and $5.56mg;hr.ml^{-1}$(doxycycline).