In situ hybridization with a 3H labelled probe on cryosections from 6 spleens of HIV I sero-positive patients with thrombocytopenic purpura showed the presence of HIV RNA in 4 of the 6 spleens at the follicular hyperplasia stage. Two patterns of hybridization were observed: first, a diffuse autoradiographic signal, displaying an irregular network, detected in 1 or 2 germinal centres (GC) per section (17%); secondly, the presence of very few distinct radioactive cells in the labelled GC. A similar pattern was observed in an ARC (Acquired immunodeficiency syndrome-Related Complex) lymph node, but with a more intense and frequent hybridization signal. These results indicate that the spleens, like the lymph nodes, are involved in the course of HIV infection but with a less intense tissue-virus interaction, which may explain the minor morphological changes observed in the spleens. In addition, a careful examination of the lymph node tissue indicated that lymphocytes are the predominant cell type infected with the virus. As for the follicular dendritic cells (FDC), a similarity of the hybridization signal observed in the GC and in vitro HIV infected cells suggests that the FDC could also be sensitive to the virus.