We have studied the immunological role of human immunodeficiency virus type 1 (HIV-1) p17 because the p17 antibody titer is high in asymptomatic patients and decreases with disease progression. Previously we found that monoclonal antibody (MAb) reactive to the p17-derived peptide 30 to 52 amino acids in length, namely P30-52 MAb, had cross-reactivity to the third variable region of the envelope glycoprotein of HIV-1 (Env V3) and also inhibited the viral multiplication of the supernatant of HIV-1-infected MT-4 cells co-cultured with the MAb. The relation between the cross-reactivity of the P30-52 MAb and the inhibitory mechanism is not clear; however, P30-52 might be useful for the development of therapeutic and vaccination strategies. In the present study, we examined the suppressive mechanism of the P30-52 MAbs, and found that the copy number of HIV-1 RNA in HIV-1-infected MT-4 cells was not reduced by the addition of the P30-52 MAbs, and the expression of RNAs of p17 was slightly enhanced 3 hr after the infection, although that of Env V3 was the same as the control level. In contrast, the expression of cellular p17 DNA and p17 protein was reduced by the addition of the P30-52 MAbs. In conclusion, the P30-52 MAbs did not suppress the HIV-1 mRNA level in the infected cells, but might inhibit DNA synthesis, and consequently bring about a reduction of p17 protein synthesis and a decrease of infectivity of the supernatant. The results demonstrated that the P30-52 MAb could be used as immunotherapeutic substance for HIV-1.