SUMMARY: Plasmodium berghei yoelii (p.b.y.) was found to cause an acute self-limiting infection in Balb/c mice, lasting for 14 to 18 days. A sharp fall in the primary response to sheep erythrocytes, as measured by the number of haemolytic plaque-forming cells in the spleen, and by the appearance of antibodies in the serum, coincided with high levels of parasitaemia between the 8th and 11th days of p.b.y. infection. A secondary response to sheep erythrocytes was similarly affected when animals were infected with p.b.y. 9 days before the second antigen injection. Mice were resistant to reinfection with p.b.y., which produced either transient or no parasitaemia, and no immunodepression.