We have tested seven pairs of primers for the detection of HIV2 DNA by single PCR in positive cultures from 21 infected patients. Four of these primer pairs were then used in a comparative study of single and nested PCR for the detection of HIV2 in fresh lymphocytes from 33 patients infected by the virus. HIV2 DNA was detected in 17 of the 33 patients (51.5%) by single PCR and 29/33 (88%) by nested PCR. All the patients negative in both nested and single PCR were asymptomatic and had CD4+ lymphocyte counts of at least 500 x 10(6)/l. This lack of PCR sensitivity for the detection of proviral HIV2 DNA in fresh lymphocytes cannot be totally attributed to genetic variability and may be related to a low viral load in asymptomatic HIV2-infected patients.