The possible immunosuppressive properties of the combination sulphamethoxazole-trimethoprim or either of the agents alone were studied in healthy human volunteers. The effect of the 4-day drug treatment on the antibody synthesis after vaccination was measured by titrating tetanus antitoxin, mumps antibody and Salmonella Hb antibody levels 4 and 5 weeks after vaccination. No significant differences were found in mumps or Salmonella antibody titres in the drug-treated group compared to controls. Tetanus antitoxin levels, however, were significantly lower than in controls after the combined drug. When a fourfold or greater increase in tetanus antitoxin titre was obtained in 75% of controls, the respective figures were 38% in the sulphamethoxazole-trimethoprim-treated group (p 0.05). The implications of these findings are discussed.