The invention concerns soluble and antigenic HTLV p24 variants that can be fused to chaperones and their use in diagnostic applications such as immunoassays for detecting antibodies against HTLV-I or HTLV-II in an isolated biological sample. In particular, the invention relates to a soluble HTLV-I or HTLV-II p24 antigen comprising either the N- or the C-terminal domain of p24 and lacking the other domain. Moreover, the invention covers recombinant DNA molecules encoding these HTLV-I and -II fusion antigens as well as their recombinant production using expression vectors and host cells transformed with such expression vectors. In addition, the invention focuses on compositions of these HTLV p24 antigens with HTLV gp21 antigen and on an immunoassay method for detection of HTLV antibodies using the antigens of the invention. Also the use of HTLV p24 antigens in an in vitro diagnostic assay as well as a reagent kit for detection of anti-HTLV-antibodies comprising said HTLV antigens is encompassed.