Superposition of two unrelated processes, namely terminal reticulocyte differentiation and synchronous plasmodial development, takes place in experimental infections of Plasmodium berghei. The first process is shown to be responsible for the appearance of some discrete restriction bands of host origin when DNA is extracted from leucocyte-free blood containing synchronous parasites at early stages of infection. These discrete DNA fragments cross-hybridize with host cell mitochondrial DNA. Purification steps are suggested to reduce this effect, which might be relevant also in the case of other plasmodial species exhibiting preference for reticulocytes as host cell.