The relative frequency of peripheral blood T and B cells and their biological function(s) from a group of patients with pityriasis rosea (PR) was investigated during the acute and convalescent phases of the disease using rosetting, immunofluorescent tests, and in vitro cell culturing with anti-delta and anti-mu antibodies and phytohemagglutinin (PHA). The total number of immunoglobulin (Ig) bearing cells was significantly increased, in conjunction with a slight decrease in the T cell population. Lymphocytes with surface IgD, IgM, or both Ig, accounted for the increase in the B cell population. This increase was transient, since it was only observed during the acute phase of the disease. In spite of the increase in IgD/IgM bearing B cells, the mitogenic responsiveness of B or T lymphocytes to anti-delta, anti-mu, or PHA was similar to the same patients during the convalescent phase, or to normal donors. Similarly, the levels of serum IgM, IgD, IgG, IgA and IgE in PR remained constant and at a normal concentration throughout the experimental period. The significance of the transient increase in IgD/IgM bearing cells in the pathogenesis and etiology of PR and its possible impact on the immune system is discussed.