Guinea-pig antisera containing IgG1 antibodies have been prepared either with killed mycobacterial in paraffin oil or living BCG or intradermal injections of fractions of unheated culture filtrate. Sera, found to contain antibodies specific for different mycobacterial antigens are used in passive cutaneous anaphylaxis (PCA) in 12 times the dilution-to-extinction titer. Animals prepared with sera respond in all sites when unheated culture filtrate, strain H37Rv, is injected intravenously (i.v.) at 17-20 hours. Injection of diagnostic tuberculins i.v. into parallel recipients, in selected amounts, reveals, through reacting sites, the presence of the corresponding antigens and some information on relative amounts. Tuberculins of PPD type are precipitated from heated culture filtrate by ammonium sulfate or trichloroacetic acid (TCA) or benzoic acid. Old Tuberculins are made classically by long heating and evaporation. In all products heat-labile antigens are absent or present in small amount; yet, PPD-S was found to possess five antigens. In all PPD products, there was one dominant antigen, absent from Old Tuberculins. The dominant antigen of OT's was excluded from the TCA-PPD's but was present in most ammonium sulfate-type PPD's. It can be concluded that diagnostic testing made with OT detects a particular delayed-type sensitivity that is different in specificity from that which reacts to TCA-PPD's. Many but not all of the ammonium sulfate PPD's test for both specificities. Several different delayed-type sensitivities are acquired during infection. Supported by Grant AI 01258 of the National Institute of Health.