The reactions to and antigenicity of two human-rhesus rotavirus (RRV) reassortants (human rotavirus strain D x RRV and human rotavirus strain DS1 x RRV) with the VP7 neutralization specificity of a serotype 1 or serotype 2 rotavirus were evaluated in a placebo-controlled double-blind trial in 116 1- to 5-month-old infants in Caracas, Venezuela. The children were randomly divided into five groups to receive orally the following inocula: (i) 10(4) PFU of D x RRV reassortant; (ii) 10(4) PFU of DS1 x RRV reassortant; (iii) 10(4) PFU of RRV; (iv) 5 x 10(3) PFU of D x RRV and 5 x 10(3) PFU of RRV; and (v) placebo. The children were examined daily for 7 days following vaccine administration; 8 to 26% of the vaccinated infants developed a mild febrile reaction which in most cases lasted only 1 day. Seroresponses to rotavirus were observed in 39 to 65% of the vaccinees by plaque neutralization assay and in 57 to 88% by an immunoglobulin A enzyme-linked immunosorbent assay. Vaccine shedding was detected in 53 to 86% of the vaccinees. Analysis of neutralization antibody responses indicates that the VP4 protein represents an important component of the response induced by the vaccines.