Experimental allergic encephalomyelitis was induced by sensitization with guinea pig spinal cord in complete Freund's adjuvant with increased content of myobacterium in Hartley guinea pigs between the first and 28th day after birth. The animals either died during acute EAE or showed chronic progressive, chronic relapsing or chronic EAE with delayed onset. In animals immunized between the 1 and 13 day after birth, predominatly ordinary and chronic progressive EAE was noted, whereas animals sensitized between the 14th and 23rd day frequently suffered from hyperacute or chronic relapsing EAE. Immunization on the 24th day led to very high mortality in acute EAE. The relation of the pathohistologic alterations to different types of human inflammatory demyelinative disease is discussed.