The study was designed to find methods to reproducibly propagate human rotaviruses from fecal specimens and to determine the relationship between particle numbers and infectivity. Growth of virus was initially compared in primary and continuous lines of monkey kidney cells. Primary cells (African green and cynomolgus monkey kidney) supported virus growth directly from fecal specimens much more efficiently than continuous lines of African Green (CV-1) or rhesus (MA104) monkey kidney cells. Rotaviruses were grown in primary cells from 14/14 fecal specimens of different individuals collected over a three-year period. Although rotaviruses in fecal samples could not always be grown in the continuous cell lines, two passages in primary cells appeared to fully adapt the viruses for propagation in the continuous cell line tested (MA104). The efficiency of rotavirus growth was quantified with five of the fecal isolates. It was calculated that, on the average, 1 out of every 46,000 passages in primary cells, an average of 1 out of every 6,600 progeny virus particles appeared to be infectious. Thus, rotaviruses in fecal specimens were consistently grown in primary cells and passage in these cells both increased virus infectivity and adapted the viruses for growth in continuous cell lines.