Because of a published report that serum immunoglobulin M (IgM) was elevated in some cases of retinitis pigmentosa (RP), a genetic study was undertaken to determine if a particular pattern of inheritance was associated with elevated IgM. Seventy-five RP patients in 37 families were ascertained and classified by pattern of inheritance into six groups. Serum IgM was quantitated by the radial immunodiffusion technique. Fifty-one controls were also studied. Methods of adjusting the IgM values for sex, age, and race were derived, but only a sex adjustment was used, since no effect of age on IgM was found in this sample, and there were few data on race. There was no signifcant difference in mean IgM levels between RP patients and controls. The variance of the RP group was slightly greater than that of the controls. One-way analysis of variance of the six genetic categories of RP and controls was performed on the raw data and on the data after log transformation. The F-ratio was not significant, indicating that there was no difference in serum IgM levels among the seven groups. Possible reasons for the discrepancy in results between this study and the previous report are presented.