In vitro experiments were conducted to assess whether bedbugs (Cimex lectularius and Cimex hemipterus) and mosquitoes (Aedes aegypti formosus) could act as vectors of HIV. These insects engorged through a membrane on a blood-virus mixture. Female bedbugs were larger than males and took larger blood-meals when fed to repletion. It was determined that the full blood-meal of a female bedbug contained 0.09 x 10(5) tissue culture infectious doses (TCID) of virus and a male 0.07 x 10(5) TCID, while partial meals taken when feeding was interrupted contained 0.013 x 10(5) TCID and 0.015 x 10(5) TCID for female and male bugs, respectively. Reverse transcriptase activity was assayed after culture of insect extracts in H9 cells: this showed survival of virus in C. lectularius for up to 4 h, in C. hemipterus for up to 1, possibly 2 h, but no survival in Ae. aegypti formosus. Four attempts to transmit the virus by interrupted feeding by C. lectularius from a blood-virus mixture to uninfected blood failed. It is concluded that Ae. aegypti formosus and probably other mosquitoes are not mechanical vectors of HIV and that such transmission is also unlikely to occur in bedbugs under natural conditions.