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The invention provides cryst. minocycline base. particular, three cryst. polymorphic forms, designated Form I, Form II and Form III, of minocycline base are provided. These are characterized by XRD and IR data. Processes for prepg. the new polymorphic forms are also provided. For example, Form I is prepd. by dissolving and/or suspending amorphous minocycline base org. solvent chosen from ethers followed by crystn. from the mixt. Thus, Form I of cryst. minocycline base was prepd.: amorphous minocycline base (0.5 g) was suspended Me tert-Bu ether (4 mL); the product was filtered, washed with Me tert-Bu ether (1 mL) and dried under vacuum at about 45-50 to yield cryst. minocycline base; yield: 0.38 g; 4-epi minocycline: 0.06 % area (HPLC); m.p.: 113.