Serum thymosin-alpha 1 levels as detected by radioimmunoassay (RIA) have been reported to be elevated in AIDS. We studied 143 individuals in two risk groups for AIDS (male homosexuals and haemophiliacs) for serum thymosin-alpha 1 and antibodies to HIV. RIA for thymosin-alpha 1 was performed in Dr A. Goldstein's laboratory at George Washington University, Washington DC. We found that similar proportions of seropositive and seronegative subjects within each risk group showed elevated thymosin-alpha 1 levels. Retesting after 6 months did not reveal significant increments over previous levels in seropositive subjects or in those developing HIV antibodies in the repeat samples. Thus, although elevated thymosin-alpha 1 levels might be a possible serum marker for AIDS, their association with HIV infection was not demonstrated. In addition, the male homosexuals we studied showed significantly lower thymosin-alpha 1 levels than haemophiliacs but this difference remains to be explained.