RANTES (regulated on activation, normal T expressed and secreted) is a member of the chemotactic cytokine (chemokine) beta subfamily. High affinity receptors for RANTES have been identified on a human monocytic leukemia cell line THP-1, which responded to RANTES in chemotaxis and calcium mobilization assays. Steady-state binding data analyses revealed approximately 700 binding sites/cell on THP-1 cells with a Kd value of 400 pM, comparable to that expressed on human peripheral blood monocytes. The RANTES binding to monocytic cells was competed for by monocyte chemotactic and activating factor (MCAF) and macrophage inflammatory protein 1 (MIP-1) alpha, two other chemokine beta cytokines. Although MCAF and MIP-1 alpha competed for RANTES binding to monocytes with apparent lower affinity (with estimated Kd of 6 and 1.6, nM respectively) both of these cytokines effectively desensitized the calcium mobilization induced by RANTES. The chemotactic response of THP-1 cells to RANTES was also markedly inhibited by preincubation with MCAF or MIP-1 alpha. In contrast, RANTES did not desensitize the THP-1 calcium mobilization and chemotaxis in response to MCAF or MIP-1 alpha. These results, together with our previous observations that RANTES did not compete for MCAF or MIP-1 alpha binding on monocytic cells, indicate the expression of promiscuous receptors on monocytes that recognize one or more cytokines within the chemokine beta family.